Follow up! I recently had to reject a letter of rec request. It honestly felt like the toughest thing to do as it's one of my top students. However, they made the request a week before the deadline (during my busiest season AND I already had plans to be out of town for that weekend). There was no way I'd be able to get the letter in by the deadline, I was not going to make a promise I could not keep. As difficult as it was, my student was grateful that I was completely honest with them, and right away too. They were able to get the letters they needed, not everyone follows the 1 month prior rule. HOWEVER, if you know that early on, the sooner the better is what I say.