I work at the beginning for about 2 to 2.5 hours at a stretch, then a 15 minute break or so. This is to get the rough shape of the song in order. Usually my very first rough mix, if the producer has not heard all the overdubs, or has not had the time to edit the arrangement (typical scenario), I'll simply make a great sounding static mix for him to listen to and then make his arrangement decisions.
At the end of the mix, it might get into 1 hour or less, then a break to reset the ears, then back in again. this is where we get into 0.5 dB adjustments on certain phrases, etc. once we get to this point, it takes longer, simply because I do not want to overmix it. Usually I will do no more than a few (4-3) of these minute hair splitting adjustments, and then it's done.
When I mix with a producer, we will take turns at the last leg of the mixing, so we can both hone in on different aspects of the song. the producer might mute or unmute a part, and then I'll massage the mix to make those changes he made sound great.
OK, Back to mixing...