like waltz said, "conventional wisdom" seems to say 85db or so...which without a decibel meter handy, is just loud enough to be able to clearly hear someone talking next to you
i usually mix quite a bit lower than that. can't give you a #, but probably 65-70db most of the time in order to prevent ear fatigue. then, when i think the mix is about done, i'll turn it up as loud as i can do w/o the monitors clipping.