How long should you give a referee to write a reference letter?

What length of time should you have between the time the referee agrees to write a reference letter and the due date of the reference letter?

I am sending out packets of information (i.e. personal statement, instructions, resume, etc.) to my referees for a scholarship application tomorrow morning.

It turns out that I have to get these letters in by Sept. 21st (instead of Oct. 19th, which is the national deadline, not the internal deadline). Do you think this is enough time, or am I being kind of a jerk with the time constraint? To clarify, this means that the letters would have to arrive at the office by that date, not be sent out.