g1smd - 6:44 am on May 1, 2012 (gmt 0)
I leave 301 redirects in place essentially forever, using the word "permanent" as a hint. Just last month Google asked for some URLs that haven't existed since 1998 - but which (presumably) something somewhere links to and which still bring the odd visitor. The site has had two or three changes of URL structure since then (originally .html, then two versions with parameters, and now extensionless), and the redirects get the visitor to the right page with the minimum or fuss. YMMV.