The last of the purges was in 2012. We're entering the very early stages of another that will start around October or November -- there are already accounts set to 'Account banned' (not frozen), which is what accounts become when they're set to purge.
An account must be inactive for at least eighteen months to be purged, though not every account left for that length of time will purge. I've seen accounts inactive since before 2005 that haven't purged. It's likely something similar to stuck pets, plus I've also heard that accounts with lots of customizations, painted pets, etc are less likely to purge.
Ugh I hope they don't purge one of my sides. I can't remember the pw and I think on the email it's tied to I clicked the "stop emailing me" link and apparently that's irreversible. It's nothing special but I liked my Jetsam.