I have two schools of thought here... Works for me. Sometimes I will work legs on a day by themselves as part of a 4 day cycle so it is possible for me to hit them twice in a 1 week period.
The other think I do is break it out into 3rds with my other bodyparts:
Chest and calfs, Shoulders and Quads, Back and Hams, Arms(by them self)
I also do cardio everyother day for 45 min.
What I have noticed is that splitting up my legs allows me to really beat the hell out of them. But, I only do this for a month on and a month off because by the end of a month, they just don't get enough rest. But they really cut up well during that training.