Good question. I suspect there is no one answer.
I find that it depends on how much I’ve been working out. I try to train three/four days a week depending on how I’m feeling. I hit each body part once a week. I was doing four days a week with a upper lower split training twice a week but since I’m a really hard gainer I have switched down to once a week. If I do a fourth day I do a full body workout with lower loads and/or more extensive cardio. If I’m just off a break twice a week for each part works but once a week is better (for mass gains) after I’ve been going at it for a few months.
I suspect that as one gets better at this stuff your capacity for recovery allows one to train more effectivly with a 2 times a week routine.
The best thing I’ve done for my training was to buy BRAWN. Best damned book I’ve read on training. Squats man, it’s all about Squats.