Hi I have just read this on the Disney website...
If you are an Annual Passholder, you are also invited to plan 7 days of FastPass+ selections within any 30-day period—for 1 park per day. Your 7 days don’t have to be consecutive and you can plan for an additional day as soon as one of your existing days has passed. You can choose up to 3 FastPass+ experiences per day up to 30 days in advance of your visit—so you can relax and enjoy the moment with your family and friends, knowing that some of your must-do attractions and entertainment are all set.
If you are staying at a Disney Resort hotel, you can make your FastPass+ selections up to 60 days prior to check-in. Plus, you may hold FastPass+ selections for up to 7 days or the length of your hotel stay—whichever is greater!
So - if you are onsite, you can book 60 days in advance for the entire duration of your stay.
An Annual pass holder can book 30 days in advance but can only book 7 days worth of FP+. An 8th day can only be booked once day 1 has been used.
What about an offsite guest that is not an annual pass holder. Maybe a 14 day hopper ticket holder? Can they book in advance?