It depends on the system. Very roughly (and no guarantee it will be the same for 2012):
Fundamentals - 8 weeks: M-F 8-5pm. 2 anatomy labs and 1 OMM lab per week.
MSK - 4 weeks: M - F 8-5pm with 1 anatomy lab and 1 OMM lab per week. You will get one day a week, on average, where you are out by 12.
Neuro - 12 weeks: MWF 8-12ish T TH 8-5pm 1 anatomy/Neuro lab and 1 OMM lab per week
Heme - 8 weeks: MWF 8-12ish T-TH 8-5pm Done with anatomy!! 1 OMM lab per week. On average, you'll get one day off a week when the other group has OMM
Derm - 4 weeks: ? <--- not there yet 🙂