Uh, this is from a while back and wasn't scientific, but mine ran at full brightness for around 30 minutes then started to taper off after that during the next hour. Direct drive, so it will run the batts dry, but it will become more and more dim till it's just barely visible.
1-2 full brightness hours for a space needle II--only if you have it hooked up to a bench supply /ubbthreads/images/graemlins/wink.gif
-Jason