Your best bet is to find a "timeline" thread for the "District Office" (also known as a "Field Office") that will be handling your case (i.e. where your interview will happen). Look at people's timelines on that thread and it will give you an idea of what kind of waits you are facing.
In general, PD->FP letter is 2 to 4 or 5 weeks. PD -> Interview letter is anywhere from 2 months to 2 years or so, depending on your luck. In general expect 4-6 months between PD and your interview letter.
Unfortunately, there's nothing you can study to make the USCIS test of your patience any easier. Sit back, check your mail every day and enjoy the wait.