Far-away vacations often 3-9 months in advance (to get reasonable price during the busy school vacation week). Nearby trips some weeks before. Have also done many "hey, here is a sale to <some place I'd like to visit> tomorrow, let's go", for weekend trips.

Quite often I travel for leisure to attend some event/museum special/exhibition/tour/etc which sometimes sell out, so when it becomes available (often 6+ months in advance) buy the ticket and flights. Hotel I usually book a month or so before the trip.

I find that having a vacation(s) booked even long time in the future makes me already anticipate the trip and makes me happier.

A few times had to cancel vacation plans due to important work commitments coming up, but I just let it go (after canceling anything refundable). It is cheaper to buy non-refundable and sometimes let it go, than booking refundable tickets (same for work travel).

For the school vacation weeks I do agree in advance (even 12+ months in advance) to have vacation from work that week.