Can't imagine it'll be any time soon.
The 2009 is very similar to the 2010. They're not going to EOL the 2010 anytime soon. They're both full 64-bit compatible machines, and they run fairly standard hardware. From what I've heard from Apple internally, XNU isn't changing that much anymore so the driver interface for OS X is relatively stable at the moment. That being said, it doesn't take them much effort to maintain the existing kernel extensions for these machines, so there's no real reason why they would drop support for them.
Personally, I'm still running 10.8.5 on my 2010. Works fine, does everything I need it to. I don't really care if it's not getting updates anymore, because my home network is firewalled and I'm reasonably careful on the internet plus I have backups (and backups of backups).
I guess what I'm trying to say is that you probably don't need to be running the latest software to have a useful machine. Apple would, of course, like you to believe otherwise, because that's what makes them so profitable. Life does get exceptionally simple once you put your foot down and decide that you're happy with what you've got (assuming it does everything you need it to, which my machine does).
-SC