How deep can you go without worrying about getting back to the surface with insufficient oxygen? ("inquiring minds want to know")
It's not air consumption that is usually the problem. It is usually depth and decompression time...however, that could lead to an out of air situation. Here's why:
The deeper we go, the more nitrogen from the air we breath is pushed into our bodies. Recreational diving is based upon bottom times that allow the diver to make an ascent without the need for decompression stops along the way to "off-gas" nitrogen. Here is a typical "Dive table" for calculating allowable bottom time: http://www.tdconline.com/tables/table.htm
On average, the max depth for a recreational diver is about 130 feet. This allows only about 5 minutes of bottom time. After that, you must start heading up at a rate not to exceed 30 feet per minute. At this depth, it would take over 4 minutes to surface. Most divers can get 50-80 minutes on a tank of air. So as you can see, a on simple "down and back" dive from 130 feet, you would probably not run out of air. You must always surface with at least 500 pounds per sq. inch (about 50 bar) of air (pressure) in your tank. A typical tank is aluminum and holds 80 cubic feet of air at 3000 PSI.
In considering air consumption however, you must take into consideration that as you descend, your air consumption increases dramatically. This is due to compression of the air as you experience increased compression from water pressure. Each 33 foot decent equals one "atmosphere" of pressure. Boyle's law dictates the compression rate and if you are a real "geek-a-saurus" you might find this article interesting: http://www.mindspring.com/~divegeek/estimating.htm
Essentially though, at 130 feet, you are at 5 atmosphere's and will use air at a rate 5 times greater than that at the surface.
Say your surface rate is 25 pounds per minute (probably a decent average), and we want to know how fast will we use 2000 pounds (2/3 of our tank) of air at a depth of 75 feet. Here is an equation that works for calculating air use:
SAC - Surface air consumption
DCR - Depth Consumption Rate
SAC Rate = (DCR x 33) / (Depth + 33)
DCR = SAC Rate x (Depth + 33)/33
Dropping our numbers into the handy equation we get: DCR = 25(SAC) x (75 depth + 33)/33 or DCR = 25 x 108/33 or DCR = 81.81
This means at a depth of 75 feet, we will use 81.81 pounds of air per minute. Dividing this into the 2000 pounds, we see this amount of air would last 24.4 minutes.
At 132 feet this comes out to DCR = 25 X 165/33 or DCR of 125 pounds per minute. Dividing the 125 pounds into 2000 PSI we would have about 16 minutes of air at 132 feet. That would (should) leave enough to make a controlled ascent to the surface. However, if a diver exceeds the recommended "no decompression" time guidelines then he must make stops at various higher depths to allow nitrogen to exit the tissues. If the bottom time results in deco stops of 10-15 minutes (and they could) then the diver has a problem and a decision. i.e. run out of air and die, or surface after decompressing as long as possible, surface and possibly risk decompression sickness (the bends).
The lesson here is to never, never, ever exceed your limits or your abilities.
OH...BTW, we all carry dive computers that calculate and adjust our allowed bottom time so that we can stay within the limits. We also keep a close eye on our air pressure gauge to make sure that we are not exceeding our limits. We start for the surface with 750 pounds and should be at the surface with 500. All dive durations, no matter the depth consider air usage first and foremost. Some dive computers are "air integrated" and can actually estimate "time to out of air". However, there is no substitute for conservatism, safety, knowledge and experience.
This is WAY more than you probably wanted to know. If you take a scuba certification class you will learn all of this.