A while ago, there was a friendly argument in this thread about how long the oxygen supply in a U-boat lasted before it needed to be augmented with compressed air, and how long the compressed air supply lasted.
I have now received an answer from an officer who once served on U-boats, see here:
http://www.subsim.com/radioroom/showthread.php?t=185993
His personal answer provides, I think, unique knowledge from a rare survivor with direct expereince.
Stiebler.