This is a question which really depends on what your goals are and how much time you have to spend in the gym. The good news is to see benefits from weight training you do not need to have long sessions as a couple of exercises performed correctly and at the right intensity can create enough stress on the muscles to enhance adaptation.
The main areas I see people go wrong is that the training is not consistent enough, the technique is not correct and the intensity is either too great or light. Also the variety of exercise is not changed enough which allows the body to get used to it.
Going back to the question of how often should I train? This requires breaking it down into whether you want to just stay toned and keep good strength or if you want to actually build muscle. Although similar rules can apply.
Muscles are made up of three types of fibres known as fast twitch, slow twitch and intermediate. Fast twitch are recruited when the load is heavy or when explosive movement is performed but they will fatigue quickly. Slow twitch can keep going but are not used so much during heavy loads as they are for endurance,and intermediate fibres are in between and adapt to the type of training you predominantly do.
I believe a good weight training program should encourage all fibre types to be trained.
To be continued. Sorry I didnt answer the question.