I don't get many but I usually respond to most. The exceptions to that are the few people that I know and speak to outside DA. Saves me from posting a "thanks" that no one else is really interested in reading anyway.
I don't really get that many. but to the ones I do get. If they ask a question or say something interesting I reply. But if they're something like "OMG THIS IS SO COOL" and just excitement I don't reply.
I reply to them once they aren't super short things like an emoticon or something. I just hate comments like those... no offense intended to those who do that or anything... D: I just don't like them out of personal experience lol.