They usually do the preliminary room assignments about 5 days ahead of your check in date, so as long as you get them in the week before you're fine. That said, I tend to have the notes added to my reservation as soon as I think of it because otherwise I'll forget to do it at all LOL.
If you have more than one request, it is recommended that you prioritize them, and make no more than 2 or 3 total requests with the understanding that you may only get one of them. for example:
1st priority: Ground level
2nd priority: no connecting room
3rd priority: building XYZ
This helps the CM's making the room assignments know what is MOST important to you, so that if they can only meet one request they know which one to shoot for.