If you have been around the Iron Game for a while, there is one name that comes instantly to mind when people discuss leg training. That name is Tom Platz. "The Golden Eagle" is widely regarded as having the most massively muscular legs in the history of bodybuilding. His thighs measured a whopping 35 inches and were shredded to the bone. Tom's muscle building program that created those unforgettable wheels revolved around one exercise and one exercise only; the barbell squat. If you want to add some serious mass to your legs you had better familiarize yourself with this exercise in a hurry.
Since the quads typically have a very wide variance of muscle fiber types you can use a great range of reps in your quest to build pillar sized legs. Often times, Tom and other lifters with notoriously huge legs, would go as high as fifty reps per set on squats. When I was young and painfully skinny, high rep squats were one of the most effective discoveries I ever made and helped me and my brother pack on size faster than anything we had ever tried to date. The old 20 rep squat programs from the golden era of the Iron Game made a massive man of many a skinny boy. For over 15 years I have used high rep squats with hundreds of clients, and without fail they have always gained enormous amounts of size and strength; not to mention several inches of raw mass on their thighs.
Although high rep squatting leads to massive and rapid gains, I usually recommend starting with a few heavy sets in the 4-8 range first. Sometimes I even add in a set of 10-15 after the heavy sets, and finally finish with one all out set of 20 (and sometimes 30-50). When you use this rep scheme you ensure that you hit all fiber types and stimulate the greatest amount of muscle growth possible
Another note that needs to be added here is that high rep squats shouldn't be done until you have mastered squat technique with several months of low rep training. You need to build the strength, coordination, endurance and stability needed to safely complete picture perfect low rep sets of squats first before you can move on to the high rep sets
Beginners should squat three times per week, intermediates twice and advanced lifters should probably only squat once every 5-10 days, depending on a variety of factors and how much running and other extra curricular activities you participate in
When putting together your muscle building program, be sure that squats are the focus of your lower body training; if not the only thing you do. Once you have mastered "the king of all exercises" you can then begin to think about adding in stuff like lunges, step ups and glute ham raises. Until then, and until you have gained significant size and strength from a steady diet of squats, I expect you to be spending a lot of time in the power rack.