What's the MTB learning curve like?

[VW Beetle]

Glad to have found your site! A nice change from all the little boys that populate those other bike forums!

I have been riding a road bike for 3-4 years now and just bought my first mountain bike (a Rockhopper '05) a few weeks ago. I wanted to add a little variety, and I also thought it might allow me to extend my riding season a bit if I could use it for commuting on warmer winter days and in early spring. I managed to get in one ride the first weekend and then the snow hit, so I haven't been out much yet.

My question is what sort of timeframe can I expect in acquiring intermediate-level mountain biking skills? Being the impatient sort, I hate to feel hemmed in in terms of my abilities to negotiate a reasonable range of trails in the typical state forest mountain biking areas. I know that New England means ROCKS and lots of 'em, and that I'll need to gain practice navigating them, learn how to bunny hop, etc. But, if it's possible to answer this sort of question in the abstract, if I were hitting the trails consistently once or twice each weekend, are we talking weeks....? months...? or years...until one gains reasonable profiency? I am a "young" forty-ish rider, not a super athlete but reasonably fit and up for almost anything.

Thanks!