The sooner the better! You should definitely talk with your faculty member prior to submitting their contact information on the application. You need to let them know to watch out for the email! You also do not want to wait until the last minute. Faculty are very busy the last two weeks and the first two weeks of every semester, so avoid those times when possible. If it is getting close to the deadline, be sure to ask them immediately! Keep in mind faculty have a couple extra days after the deadline to complete the online recommendation, but the recommendation request must be made prior to the deadline and should be made as soon as possible.