I think its every 21 days. If the weather is the same with you as it is here, when he has done the deed you will see the mud on her back.
We find with our boar that up until the deed is done he "washes" every day in the wallow until he is well covered in brown mud. Boar version of aftershave I think.
He eats with that sow, never leaves her side, she is the most important sow in the world. Once has covered her he lies on his own, chases her off his feed and stops bathing. This behaviour has in our experience been the norm to a greater or lesser extent in all our boars.
Also the boar gets a really rancid and strong smell when he is covering. Again thats only in my experience.