@Roger311 If your booking window is unlimited, then a guest can request a reservation or book if you have Instant Book 18 months in advance. Usually not advisable to do that as you may want to adjust your rates as you gain more current market information about demand.
@Roger311 A lot can change for hosts and guests in 18 months! If you are 100% certain that your situation will not change in 18 months, then have at it. Perhaps you'll want to block off some dates down the line, perhaps you'll consider selling/moving, etc. Plenty more what-ifs to consider.
I do not receommend it. at most 9 months if you are flexible enough. The thing is guests change their minds and plans a lot and they can always try to cancel it and get a refund regardless.