If you only have one billy, get a wether as companion, and have girls out during the day and boys in the evening, if your paddock is secure and has a shelter they can wander into, I would leave them out all night.
My billy (and his wether friend) has a look-out window into the paddock, so he can watch the girls during the day, then goes out for a wee while just before it gets dark.
One of my milkers (not pregnant but running through) was strutting her stuff in front of his window this morning, even though no tail wagging or other signs of them being in season. Better not to take a chance...