You need to give us at least a month, and it may take longer depending on our workload. Honestly, letters of recommendation are low priority in comparison to abstracts, manuscripts, grants, books, grading, committee work, candidate evaluation, advising, etc. Your needs do not necessarily translate into our obligations, and if the deadline is imminent, don't get huffy because we can't get it back to you immediately - your lack of preparation doesn't translate into our emergency.
Additionally, it's good to provide supplementary materials to review - resume/CV, letters of interest/personal statements, copies of transcripts, etc. The letter of recommendation is supposed to mean something, and we put our reputation on the chopping block when we write them, so the more information we have, the better the letter we can write. Additionally, do not assume that letters of recommendation will always contain positive information. We can and do write honest assessments, including weaknesses.