How Long Does It Take For Bettas Tail To Heal?

I have a delta tail betta and he's torn most of the tip of his caudal fin after flaring for several hours. Don't think for a moment that he's a tail biter because he's not. I believe all this is from flaring because this has never happened until I placed the mirror in front of his tank for an extended period. How long does it usually take for bettas fins to grow back? Days? Weeks? Months? They're not that serious but the torn parts make him look ugly. Thanks!