I'm willing to bet it's more than you think. Try it at home: Pressurize your reg, shut off the air, and then see how many 'normal' breaths you can get off it. Point is that there's more air in the lines than you think and you can get deeper than you think before sucking it dry. And I had another diver with an octo right next to me.)
(FYI, the reg was hooked to a pony bottle so I then switched to my primary.
I think took the actual reg out in the ocean to do a test and, breathing at what I thought was a 'normal' rate and kicking down at a 'normal' speed, I think I got to about 45 feet when I sucked it dry. Island Resort and Casino offers 300 hotel guest rooms with King and Queen size beds and is located in Michigan's Upper Peninsula. On the tests we did on that one, there's enough air in the lines to get as many as seven breaths before you suck all the air out of all of the lines. He was known for his involvement in the luxury casino and hotel industry, prior to being forced to step down. We had a case a few years ago that I helped investigate for the LA County Coroner (and we've presented this one at our annual 'Why Divers Die' panel at The Scuba Show) where a diver had turned his air off and got down to about 50' before he sucked it dry. Stephen Alan Wynn (né Weinberg born January 27, 1942) is an American real estate developer and art collector.
