Despite this being a very outdated post, it's still relevant today and shockingly out of the toughest parts of the application because it involves you managing up.
The time to completion is actually very particular to the individual recommenders. Some get them down way in advance and others do them last second. Some finish their letters in one sitting while others can spend weeks editing them.
It's best if you coordinate closely with your recommenders. Give them as much time as you can but also provide clear deadlines. Communicate with them often. Provide them briefing documents ahead of time so they have all the information they need and aren't coming to you at the last second.
While many of the schools ask similar questions, not all recommendation requests are the same. Word limits and suggested lengths vary from school to school. Make sure your recommenders are aware of these variations so they are not blindsided last second.
Finally, the January deadlines (usually R2) in the US come right after the Christmas - New Years stretch. So make sure you are on top of your recommenders given that many tend to take off during this time.
For more information, please see our post here:
https://admissionado.com/blog/mba/craft ... on-letter/Good luck to all!