A phone has to last at least 12 hours not plugged in, even if that is with minimal use, screen brightness turned down and some of the instant data turned off - it would be better to get over 12 hours with heavy use though.
I find what really kills the battery dramatically is when in an area with really bad service where the phone is constantly trying to connect voice and data. My Droid will last all day with moderate use, but in an area with really bad service it can be dead in 4 hours.
I don't really see much point in guessing what the battery life of the Bionic will be - How could anyone possibly even make an educated guess at this point? Based on Motorola's prior performance I hope it will be "good enough" or I won't buy it.