Everyone is going to be slightly different based on where in the country you are and how your breeding season is going.

Ie.
If you got off to a slow start you might want to keep them cool longer
If you got an early start (and have seen good signs - ovulation, lots of locks, or they want nothing to do with each other, etc) then warm them up sooner.

Your local weather also plays a part...here in Indiana we are still expecting a couple more fronts so I am keeping them cool longer.

There is no exact science.

Hope this helps.