It takes very little time to get up and running with R, especially if you have another language under your belt already. Spend some time up front learning the basics of the syntax (particularly how to assign values to variables, how to select or subset data, the different data types, vectorization) and you can be doing basic analysis, plotting and data manipulation within hours. This website will teach you more than enough to be dangerous: http://www.statmethods.net/

Having said that, there are a few things that will trip up the beginner. I really enjoyed reading this guy's summary of several of those things: http://arrgh.tim-smith.us/

I don't think you could ever really 'master' R, since there are several thousand packages that make up the full suite of its functionality! But there is plenty of help both online (eg here are the questions and answers tagged 'R' on stack overflow: http://stackoverflow.com/questions/tagged/r) and in the documentation.

The best advice I can give you is to dive in and start playing with the language. You will be surprised at how much you can accomplish in a short amount of time. Post here any specific questions; I'd be glad to try to answer them.