Depends. I can usually force myself to finish something up. Once I get an inspiring intro going, it helps a lot in keeping me going on that specific track. The total time I spent on Devastation's Doorway was maybe 15-20 hours over a few weeks, with the majority of the time being spent doing mixing and doing sound design (tweaking drums, percussion, EQing, etc). I've become a lot more efficient since then, though, and learned some new production tricks.
Talking specifically about remixes, I can usually get a decent one and a half minute start in about an hour and a half or so. Actually, last night I found an old Mario Kart track I started of the ending theme and added another minute and a half in about an hour (I'll probably post the WIP here soon). I do most of the mixing as I'm going along and then do a final revisit once I finish the track.
So yes, if I had time I could easily finish a mix every two days or so.