I've made decent gains in the past by working everything twice a week and keeping my lifting volume in check (maybe 3 sets each of two exercises for each muscle group). But eventually I hit a major plateau, and like most people here, I found I could make better progress by hitting each muscle group only once a week and really uping the intensity. Like Thaibox, if I'm lifting really hard, I've found I need at least 5-6 days of recuperation before my muscles are ready to be blasted again.