http://www.craigslist.org/about/help/email-relay describes the semi-anonymous email relay service Craigslist runs for its users. That page states:
Q: Can I continue to communicate with respondents after my ad has been removed?
A: Yes, existing reply email communication threads can continue for up to 6 months.
Users of the Craigslist forums report that the service sometimes allows a thread to continue for longer: e.g. eight months.
Still, what happens if you try to reply when it's too late?