You should find that with a little practice, you'll touch your BC
twice.  Once to decend, then once after surfacing, to inflate for
floatation purposes.  Other than that, when properly weighted, you'll
control everything with your breathing.  Relaxing helps too, but this
is where experience comes in.  This will also come about the same time
you find that your breathing efficiency increases your bottom time.

I have read kjw's comments and have not reached his nirvana of touching my
BC twice except if it is a strictly single level dive, I find that I mostly
uses breath control for bouyancy control but adjust the amount of air in my
when I move large amounts in depth levels say ( and one does it fairly
subconsiously ) 3-4ms. I find it also makes a difference the thickness of
the wetsuit, the thinner the wetsuit the easier it is to control and the
less fiddling with the BC another good reason for warm water diving.

kjw is correct the more experianced and the more confident you get the
easier it becomes and the less air you put into and dump from your BC the
more you have to breathe .

You shouldn't have to put air in your BC at that depth.

You did.

Your question can't be answered effectively. There are too many possible
variables not known. The most significant of them is probably how much
compressible gas you are carrying. This involves such things as what kind of
thermal protection you're using - wetsuit, drysuit, nothing? If wetsuit, how
thick, what material and how old all matter. Wetsuits compress and expand as
depth changes, changing buoyancy as they do. The thicker they are, the more
variation you're likely to experience.  Different materials expand and
compress at different rates and old material tends to compress quicker than
new material. Of course the gas in a drysuit expands and contracts too, but
not at the same rate as a wetsuit.

You do the best you can to get your weighting as "right" as possible, and
then fine tune it from experience. When you've got it as correct as
possible, you'll learn how changes in equipment and changes in depth affect
your buoyancy by experience as well as how much buoyancy shift you can
adjust for without adding or bleeding gas from your buoyancy control device.

I'm a warm water diver. I don't normally use thermal protection. I don't
carry anything compressible. My only buoyancy issue is the gas I breathe
during the dive. With one of my 80 cubic foot tanks, I start slightly heavy
and end slightly light, but am never so far from neutral that I can't adjust
simply by changing breathing patterns. As a result, I often do an entire
dive without adding any gas at all to my BCD. Since I have nothing that
compresses, I can move from the surface to any depth I like, without a
significant shift in buoyancy. It's a very effortless way to dive. I don't
do quite as well when using my 100 cubic foot tanks, but I come close. The
100s are a bit heavier at the beginning of the dive. They're not so heavy
that I can't overcome their negative buoyancy by finning a bit, but if I
want to maintain neutral buoyancy, I add a bit of gas at the beginning of
the dive. While that gas is in my BC, I get a buoyancy shift as I change
depth. Because I don't put much gas in my BC, the shift is small. When I've
used enough gas to overcome the slightly lower buoyancy of the tanks, I let
the gas out and I'm right back where I started with my 80s.

So, rather than suggest how much you can descend or ascend without changing
the amount of gas in your tank, I suggest you concentrate on getting your
weighting as close to perfect as possible. Spend the time to do a dive
specifically to work on your weighting and it will pay off in all other
dives. You're goal is the least amount of weight that will allow you to
remain neutral a foot or two below the surface with your tank as empty as
you normally expect it to get, with no gas in your BCD. When you've got your
weight as close to perfect as possible, you'll have reduced the effect of
depth changes as much as you can. From there, you learn to do what you have
to to remain neutral by experience.

Good Luck.

Lee

It's not really a question that can be answered in the abstract.  With
my massive experience of 20 dives <g> I can tell you that it's a fun and
useful skill to practice.  I guarantee that at least once you'll find
yourself inexplicably and uncontrollably drifting upward and/or headed
for a hard landing in the sand.

Avoid the temptation to keep fiddling with your inflator and dump
valves.  Take a couple of minutes after you first descend to get your
bouyancy in the sweet spot, and use your breathing as much as possible
for fine tuning.  If you find yourself breathing high to keep from
sinking, it's time to add a little shot of air to your BC.  When you're
just sipping air to keep yourself from ascending, it's time to dump a
little air.

Carrying as little weight as possible helps, for theoretical reasons
that make my head hurt.  But it's true.  And inexplicably, after a few
dives you'll need less weight.  I needed 32 lbs for my checkout dives,
now I'm down to 26 lbs.  This may have something to do with carrying a
ton of gear across rocky beaches, sweltering under the August sun
wrestling into a 7mm jumpsuit, and the resultant loss of hard-won beer gut.

You have explained it right to the limits of your knowledge and
experience.

I've read the other comments and I've got a couple of points to add.

1) in beginning courses, since the students are often waiting their
  turn to demonstrate skills, the instructors often intentionally
  overweight students by about 1-2 Kg (2-5 lbs) so they can stay
  firmly on the bottom.

2) The shallower you are on a dive, the more you will need to
  adjust the volume of air in your BC. (The percentage change
  in pressure from 15ft to 0ft (surface) is greater than the
  percent change from 15ft to 30ft and MUCH greater than the
  percent change from 45ft to 60ft(Assuming that you will be
  qualifying to the standard OW depth of 60ft/18M)

3) If you are overweighted, there will be a greater volume of
  gas in your BC that will be affected by the pressure change.
  Thus more need to adjust.

4) If you are diving in salt water, the change in pressure for
  a given change in depth is somewhat greater, but probably not
  enough to get too excited about. (Although the few times I
  have dived in fresh water with my standard kit, I've found
  myself leaving out 10-15% of the weight I'd use in salt water.

4) PADI says to adjust for neutral buoyancy at the start of a
  dive. I've always had a problem with this. The real "crunch"
  for buoyancy, IMHO, is at the end of the dive, when your cylinder
  is nearly empty and you need to hold a depth of 5 metres (15ft)
  for a safety stop.

  I believe that the better solution is to adjust for neutral
  buoyancy at 5M with a nearly empty cylinder and no gas in
  your BC. (Sorry, I'm going to have to go metric here since
  metric is the units I learned to dive with. But a litre of
  air weighs 1.3 grams so a 12 litre cylinder at 240 bar (100CuFt)
  that has about 3000 litres of gas in it will will change
  weight by about 4 Kg or 9 lbs in the course of a dive. If
  you are precisely neutral at the start you will be positively
  buoyant at the safety stop. (OK you should not have an empty
  cylinder - but the closer you are to empty, the more buoyant
  you will be.) Some of this is compensated by compression of
  wet-suits, etc. But I would suggest that this is not enough.

But, at the start remember that most beginners are somewhat
overweighted in any case and don't stress about it too much. You
should be diving with an instructor and possibly also a divemaster
who are likely to be carrying a bit of extra weight to be able
to give some to a student who is having trouble on a safety stop.
(I know when I'm diving around students I typically dive 2-4Kg
overweight so I can help, if necessary.)

Finally, just to clarify, I typically dive temperate climate salt
water (9-18 deg C 48-64 deg F) So I'm using a two piece 5mm wetsuit
in summer and a dry suit in winter. Buoyancy control in a drysuit
is slightly easier even though it is an extra airspace that expands
and contracts with depth. (A drysuit has an automatic dump valve so
that, in theory, it should be more or less constant volume on an
ascent. On descent, I simply add enough gas to prevent a squeeze.)

-Don