It kinda depends. I'll just fly through some tracks I do, while others take much longer. I did my section of the Global Empire collab (about 2 minutes worth of music, plus the solo) in two afternoons, with another session devoted to final mixing and mastering.
Aqueous Transgression was spread out over about a month of work, but again, in short chunks of time (as dannthr alluded to, would be a common complaint of most mixers here.) More recently, I moved very quickly through the track I did for Blind's project (like three or four days), but that's pretty uncommon for me to finish a piece that quickly.