i remember on interview day they noted that it used to be 12 hour shifts then a group of residents voted to change it and did, then those residents left and everyone else has been happy with the 12-hour shift system.
as for specific numbers of shifts i asked a resident at Stanford and this was answer i got:
for hours per week the rough guidelines averaged out over 4 week blocks:
R1 56-60 (usually five 12 hour shifts, two days off per week)
R2 54-56
R3 48-56
Stanford is a all 12's
Kaiser is 12's during the day and for overnights it's 10's for R2,R3's, 11's for R1's
Valley is all 10's,
SFGH is all 12's. (this is obviously changing since SFGH will no longer be a major component of the curriculum at Stanford)
It's a lot of hours, but people have voted to work more 12's in exchange for more days off (average of 20 shifts or less in 28 day period)