Depends a lot on the letter writer, but I did get a letter from a September rotation. My understanding is that this writer does a good deal of letters and has them all completed within one week of being asked. I'm guessing this is not normal, so asking for a letter in late September is usually pushing it. Definitely don't expect to have letters written within a week. I didn't ask for any beyond that point because I had what I wanted, but I would say any letters you get in late September or October should be considered supplemental to a complete application. That is, it can never hurt to get them, but you need to have the required amount of letters in your file before then.
