I would recommend that when you begin you don't treat the legs any different and give them the same amount of volume as you would the rest of the body. For a complete beginner 9 sets of squats per week followed by 9 sets of either PC's, GM's or DL would not be unreasonable.
Once you've been training a while you can get a feel for how much volume you need/can cope with. Use that as a guideline. However this requires prior experience of 6-12 months+ to really be in a position to answer that question properly.
This is where you will see individual variation in Full Body Routines. My own FBR was a progression from a bare-bones basic 3 times a week approach to now: http://muscleandbrawn.com/forums/ful...tml#post141495
I have tailored that approach to what I can realistically handle for each bodypart.
That varies for each bodypart, it's all about getting to know what you're capable of and what you best respond to.
Hope that helps.