Hi,
@Sailor Steve: I know that in SH4 there is a bug with the CO2 dial, but I have never seen such an issue in SH3.
@fastfed: There is a lengthy discussion of this issue in the original thread by H.Sie. I also recommend to read H.Sie's readme. The duration depends on the crew size and also on the status of the boat. For the max. duration you should rig for silent-running (no repairs and no reloading).
If you don't like the numbers, just adjust them to your likings via the Options Selector.
Regards, LGN1