They don't expire. They used to say something like 6 months, but they never actually deleted anything after the 6 months and now they're just saying:
Whoa, that's awesome. Also, I know I searched their FAQs before and never found that answer... thanks!
See, I have Demon's/Dark Souls PvP builds that I'd lose if my PS3 YLOD'ed, even if I had backups. This happened to me last year. The saves are locked, so a backup restore didn't bring them back. I have two PSN accounts (because Demon's Souls only has 4 save slots and I have more than 4 characters), so I bought PS+ on my main for the free games and all, and I did that silly PS Home Quest to get 1 free month of PS+ on my alt, which I used to copy my D-Souls saves.
Now it'll expire in one month, and that's fine, but if my PS3 dies, I can always buy one month of PS+ to redownload my lost saves. Woo!