It's an automatic 10 day window, but you can request an exception if there is a special circumstance (lost mail would certainly be just such a circumstance.)

http://ssa.gov/pubs/10090.html

"How much time do I have to appeal?

You have 60 days to ask for an appeal of our decision to stop your disability benefits. The 60 days start after you get our letter telling you that your benefits will stop. We assume that you got our letter five days after the date on it unless you show us that it took longer for you to get the letter.


What happens if I miss the time for an appeal?

If the 60-day period has passed, and you want to appeal our decision, you should tell us why you were late. If we decide you have a good reason for being late, you still may be able to appeal. For example, we may find that you have a good reason for being late if the forms we sent you did not arrive on time.


Will my payments continue during the appeal?

If you ask for an appeal within 10 days after the date you receive our *letter, you also can request that your payments continue while the appeal is being decided. If you are late in *asking for your payments to continue, you still may be able to get payments *during the appeal if we find that you have a good reason for being late."