andy i've gotten over 6 hours in an m1 on lithiums, running mic-in or digi-in, then playing back thru line out.
one thing to consider - the M1's battery meter is basically useless with lithiums. it will go from 3 bars to two rather quickly - first hour or so - then will go down to one bar within three hours, then will not ever flash at one or give the Lo Batt message - it will just go from one bar to DEAD without warning.
i used to run lithiums in the m1 all the time, and would usually get a whole show with the light on, rewind 2 60s, and listen to the whole show on the way back / in the room, and then get a first set the next night