Depends on the source. AM is very quiet, I listen in the 30-40 range while driving. Then if I switch to FM or CD-RW/FLAC I have to drop it about 20 points to not blow my ears out on switchover. So, I'd say in the 20-30 range normally. For rompin' music while overcoming freeway speed road noise, 35 is where it's at, sometimes heading up to 40. I've dialed it up to 60 (max?) during testing, but there's been no need to go anywhere near there in real listening.
Note: None of the above takes into account the addition of the subwoofer with gain control next to the steering wheel. That's adjusted separately and should be taken into account with the above numbers.
Nothing beats your own ears with your own music though. =]