Hi mate, unlike dogs, which cycle in and out of heat, female ferrets will come into heat and then experience a prolonged season, typically staying in heat until something happens to stop it, whether being mated to a intact hob, a vasectomised hob, or given a hormone injection.
If left in prolonged heat, the jill will become more prone to picking up dangerous infections and the elevated hormone levels can cause a suppression of bone marrow production, which is bad news.
A lot of people deal with this by breeding their jills every year, but I think that there are much simpler options, ones