Does anyone have a feel for the length/amount of preparation time for someone who has a long OO background and a few years of professional Java experience?? It seems that I've read that the exam covers some areas that even a seasoned developer might not hit regularly...
As a "seasoned developer," I imagine you could be ready to pass the exam in a few weeks time. Certainly, there are some things on the exam that you're like to have never used before - everyone's experiences with Java differ somewhat. However, your basic knowledge of the language will carry you a long way.
The thing I'd suggest giving yourself a little extra time for it studying "how" to take the exam. Taking the SCJP exam is quite a bit like turning yourself into a compiler. Often, you end up performing tasks that you'd normally rely on the compiler or IDE to do for you. If you're used to automated tools to do the "nitpicking" for you, it might take a little bit of time to get ready.
I'd suggest going through a mock exam or two (Dan Chilhom's are excellent) and see how you score. That would be an excellent guage of just how far you need to go to be ready.