They don't necessarily get "frigid," as they will be willing to breed; however fertility begins to decline at 75°F. with them becoming temporarily sterile at around 85°.F The more you use the buck, the longer he will maintain fertility. But after a week or so at 85°+with no breeding they typically become, what rabbit breeders call "heat sterile."
Of
course that's not a "magic number" for all bucks. Environmental variables and genetics will also play a part in the buck's fertility.