Royal Navy and British Army (and prob RAF too) use STASS (Short term air supply system). An air bottle with a divers style regulator attached to our LCJ/LSJ (Load carrying jerkin/life supporting jerkin when fitted with a stole). Its a smallish alloy bottle (about 1/2 a ltr I think) with a diver type regulator filled with pure air. I know of several Navy pilots and crew who have had to use the system due to 'falling off the deck' or ditching and who have survived. We train with the system on a biannual basis (in the Army) and it is a tremendous confindence builder. We do however use it as a secondary means of survival. We rely on using HUET techniques first and STASS as a secondary if we cannot escape entrapment by the usual means. To me it gives me a 'warm' feeling knowing if i cannot get out straight away due to disorientation or snagging problems, I have a bit of a reserve to allow me to survive a bit longer until I can. I know of a Naval Lynx crew that got washed off a deck in horrendous weather and remained calmish as their cab sank, waiting for it to settle, as they knew they could use STASS. When it all settled down, they went onto STASS and emerged upto the surface.
STASS duration. I have seen people in a contolled situation last more than 10 mins on one bottle. I have however seen in the same situation one person drain a bottle in about three gulps!!!