I get this question a fair bit. How long, exactly, is an email available to be read after it enters the system?
Its hard to answer because its literally dependent upon the incoming rate at which email arrives. In effect, new email "pushes out" old email. That's a bit simplified as some email is thrown away based upon spam rules (if you get 100,000 emails with the same subject, you can probably say they aren't all going to be read).
So, whats the average? This weekend, email was lasting around 10 hours before getting "pushed out" of memory. I've seen that jump down to an hour - but mostly its around 5-7 hours.
I'll note that the primary Mailinator use case is that people tend to read email soon after it arrives. In other words, if email only lasted 15 minutes, we'd actually fill the needs of many users.
(I'd be *very* interested in hearing ways you use Mailinator that need mail to stick around longer.)