It depends on when you're going. I've seen several rumors to expect a full roll out in October. If you're going before then, you'd have to be staying a resort that is participating in the testing. Regular fast pass is still in effect for ALL guests while testing is going on. When it goes "live", it's speculated that regular FP will be taken out but I don't know if that's confirmed yet.
During testing and when FP+ goes "live", you can only choose FP+ attractions at one park per day. If you're park hopping, it would behoove you to select your FP+ attractions at the park you will be hopping to later in the day.
If you are going after its "live", you should be able to book attractions 60 days in advance. If you're going during testing, people have reported they are able to book 21 days in advance.