As someone who has been riding road bikes forever and starting riding mountain bikes (complete rigid) in the 80's I have to say the learning curve is very individual. If you are out riding on a regular basis, and are in good health generally, skills should come quickly but it really depends on your "pucker" quota and ability to risk injury. Are you willing to take risks? I am a good intermediate rider but due to some serious injuries, I no longer push the envelop and may never graduate to the advance level, but then again, I'm happy doing what I do. I was the same way snow skiing. I had good skills but didn't want to be stressed out all the time doing things over my head.

A suggestion, ride with someone who has good skills and watch how they attack obstacles and terrain. Consider taking skills classes. Alot of areas offer them. You can check with your local MTB club. Practice, practice practice difficult terrain. Try going the log over and over. Do the same downhill over and over until you feel comfortable. Bottom line, you have to ride - it's the only way to get better.