Yep, go right ahead. The only 'limit' here is the one imposed by Reddit's own processes, where threads automatically get archived after six months. But as long as a post remains open, you can drop an answer there. Or ask OP if they'd be willing to repost the question, for more visibility (and useless internet points!). Or you can also ask the mods to mysteriously cause a question to appear (especially helpful for areas that don't get many questions).

Good luck in your answering!