I know the answer has been already given (60 days on-property and 30 days off) but it got me thinking about my experience ....
I had an interesting experience booking my first set of FP+. We will be over in a weeks time from the UK and we are staying off-property. So we had to book our FP+ 30 days out from travel. I followed all the advice that it would be busy in August so I sat on my PC at 5am (UK time) ready to book them as soon as the window opened. I was quite surprised at how much had already had been taken and how for most of the suggestions they were either early or late afternoon.