The simple answer is 'as often as you can'. If you look at all the most successful Weightlifters, you'll notice they all have one thing in common, they train a lot. The more often you do something, anything, the better your body will be adapted to performing that task efficiently and effectively. Somebody who lifts once a week has almost 7 days for his body to forget the movement patterns he spent a few hours learning one day. A person who lifts 3 days a week only has a day or two between each session to forget, a person who trains every day has almost no time to distance themselves from the movement, and a person who trains multiple times a day? Well you can work it out.
Now, for a beginner, or somebody who plans on having a life outside of lifting weights; training 12-14 times a week is a little excessive and difficult. However it is a good target to aim for and to keep in mind when you think you're training too frequently.
3x a week is the absolute minimum you should be training the Snatch/Clean and Jerk. And at that low a frequency, pretty much every session will need to be with maximal or near maximal weights, or very high volume (lots of setsxreps). Now, if you add in sessions on the days between those Maximal days, you can play with the loading of weight a bit more.