Re: my earlier suggestion, here's the relevant math:
Start by dividing your PSI used by number of minutes. For example, if you started with 3,000 PSI and ended with 2,000, that means you used 1,000 PSI. And let's say it took you 40 minutes to breathe that much, so that's 25 PSI/minute.
If you were sitting on the couch, that's your SAC rate. If you were underwater when you breathed that 1,000 PSI in 40 minutes, you have to adjust for the depth first. Calculate absolute pressure by dividing your average depth in feet during the dive by 33 and then adding 1. (As you probably remember, every 33 feet equals one atmosphere, plus there's the real atmosphere up there.) Some computers will display your average depth; if yours doesn't, and you can't borrow one that does, see if you can download your dives to a program like Subsurface, which will calculate it for you. You could also just do a square profile dive over a flat bottom to make the average depth easier to estimate. So if your average depth was 20 feet, divide 20 by 33 and add 1 to get 1.6. Then divide your 25 PSI/min by your 1.6 absolute pressure to get a SAC of about 15.6 PSI/min.
If you used the same tank for your couch SAC measurement and your dive, you can just divide the latter by the former to get your dive factor. Let's say you had an underwater SAC rate of 30 PSI/minute, and only 15 on the couch; that gives you a dive factor of 2, and tells you that you have some room to work on your technique. But if your in-water SAC rate was 30 and your couch SAC rate was 20, your dive factor is 1.5, which is pretty good. That would mean you're just a big guy who's always going to be a bit of an air hog, so you should plan accordingly with larger tanks.
If you couldn't manage to get the same size tank for comparison, or if you want a better idea of how you stack up to others, or if you're just having so much fun with all this math, you can go one step farther and calculate your RMV. First look up or calculate the tank factor for each tank you used. Divide the rated volume of the tank by the rated pressure, e.g. for a standard AL-80, 80 ÷ 3,000 = .0267. Multiply that by your SAC rate. So if your SAC rate was 30, that works out to .8 cubic feet per minute. Most guys will be somewhere in the neighborhood of .7 cf/min, at least until they get a lot of practice. Bigger guys will breathe more, and everyone breathes more in colder water or when swimming against a current. Pro tip: don't try to compete with your lady dive buddies on this; you'll only drive yourself crazy.
As to your question about how long it'll take to improve, it really depends. But I can tell you that, during my first 50 dives, my RMV averaged around .7; by the time I got to 100, I was down around .65, and by 150 I was down to a little over .5.
