If you have a voltmeter, you can tell when the Powermax drops from "bulk charge" at 14.6v to "normal charge" at 13.6v. If you really want to know it's fully charged, you'll eventually see it back off to 13.2v but that takes several hours.
Don't just take one reading and assume it's done with the bulk-charge phase, though, because early in that phase the voltage is slowly rising until it gets to the 14.6v level where it'll plateau for a while before stepping down again.
EDIT: Hehe... timing is everything.
All I really added to what idroba had to say is the voltage specs for the Powermax. BTW, the voltages I mentioned are for the Boondocker version of the Powermax converters, the ones that BestConverter sells. I think the only difference for the non-Boondocker models is a lower bulk-charge voltage.