It's going to be one of those 'it depends' answers but all it really depends on is if you're the kind of developer that tries to continually improve your skills. If that's the case then it comes down to a couple of things ultimately.
Buy WPF Unleashed (others books are available but I whole-heatedly recommend this one), work through the examples and in about 2 months or so you'll wonder how you ever got things done without it.
It's not rocket science and there are no paradigm shifting concepts to deal with. In fact, if you are used to keeping a good separation between your logic and presentation it should fit right in.
To get to a level of mastery you're probably looking between 6 and 12 months of playing with it.
Good luck.
