Finally finished. Interesting ending. For the first 90% drop I was averaging about 204 minutes though the individual levels varied widely (70 - 336 minutes). The device gives only 10% drops until the last 10%. It divides that into 5% intervals.
Well to drop from 10% to 5% took 172 minutes all by itself so I figured the last 5% would take less than that. Certainly not more. Well, it just wouldn't die!
It took another 253 minutes! Which means the last 10% took 425 minutes - just over 7 hours. If that is consistent, you could start at 10% and make it through a full workday with it on the whole time.
I wouldn't be surprised if they deliberately set the last part to be longer just because it is always best to have more reserve at the end than you expect but I don't know if that is the case.
The "Check" test which gives some stats on usage does seem to give fewer minutes than I actually get. For the stats I've been giving, they are based on those stats rather than my manual measurements - except for the last 5% because I couldn't run the check test with a dead battery.
Oh, total time was 37 hr 38 min according to the test. In reality almost certainly at least over 38 hours.