Dear Miss Snark,
Do requests ever "expire"? Common sense says (to me at least) one shouldn't wait a year+ to send the material, but how many weeks/months do you have to send in the requested material?
Technically no. Practically, yes.
First, you've got my attention when I ask for something. It's wise to leap on that opportunity with alacrity. My attention moves on as the days weeks months pass.
Second, if I'm considering you for a client, and you've demonstrated your glacial pace, I'm going to think twice about signing you. When a book is sold, the editor sends an editorial letter with changes requested. That can be 2 pages of not much, or it can be 15 pages involving a re-write. There's a deadline attached to those changes, and I want clients who understand deadlines and can meet them. Yes, deadlines can be changed. Yes, books can be postponed. No, this isn't my first, second or third choice of how to do things.
Third, markets change. If you're writing the hot new thing this summer, next summer I've seen five other things like it, and since they appealed to me, I may have signed one.
Get off your ass. Get busy. Write like it matters. It does.