I have never intentionally (or unintentionally) breathed a tank dry. (Came close once, but that's a different thread) Log book review shows Debbie and I have dozens of dives in the 40 to 49 foot range of over 60 minutes, and I had the lowest end-of-dive air left, one one occasion, about 550 psi from a 3100 starting pressure. I also noted that in my first 20 dives, (almost a900 dives ago) dives in that depth range averaged under 40 minutes, with "leftover" air of from 700 to 850 psi. My wife Debbie is a curve breaker here. After 60 to 70 minutes in the 40 to 49 foot maximum depth range, her "left over" air has never been under 1000 psi, and is usually in the 1200-1300 psi range. She is my dive buddy AND my "spare air!"
DivemasterDennis