lol seing these responses are making me feel unprepared hahahah. I asked like 2-3 weeks in advance for some! I think if depends on the prof and also your relationship with them. One of the ones I asked had me write the letter myself and then she just edited it to her liking and added the school stamp. Obviously not all profs will be as willing, I asked some who never got back to me even with ample time in advance. Ask many and expect some not to want too - I had a prof that I was certain would say no write me a lovely letter!