I train each muscle group once per week for full recovery and have gotten the best gains off of this. Anything more for the majority of people could lead to overtraining.
Controversial; unfair to assume that once a week's best for most.
IMO only, 2-3 times a week with low-moderate volume works best, based on the fact that the bod takes 2-3 days max. to recover from more frequent workouts.
This is debated by some but is my experience. The only reason soreness sometimes might last longer for some, is that (1) once a week training's more of a shock to the muscles, which leads to greater & more prolonger DOMS, and (2) once a week training for some involves high volume or extremely high intensity or a combo of both.
Even then, I can see training once every 5 days or so, because of the belief that the muscle actually starts to slide backwards when in disuse for too long a time between workouts.