I recently picked up an unfinished Yellow file and played a bit of it, I would say a couple of months ago. Unfortunately, I didn't have time to play much of it, and my gameboys are at home with my parents, so I don't play the older games that often (plus, GSC are all dead and I haven't gotten around to replacing the batteries). Aside from that, my most recent playthrough was probably a challenge a year or two ago (not on the cartridge, though). As for the last time I really played through them normally, or devoted a significant chunk of time to them, I would guess probably about eight years ago.
I should really play through them again. They're great games. I've played the remakes more recently, but still, there's something about the original charm from the older games.