Depending on your fitness levels, goals and intensity, twice a week is standard for the lower body. If you are exercising pretty hard – 3 sets of 8-12 reps for each move, where you struggle to complete the final set – you need to supply your body with a full day’s rest before working the muscles again. For this reason, I suggest either intense workouts split into upper and lower and practiced on alternating days, or a lighter, full body workout that can be practiced daily.