I sent in a quick compliment the other day about two of the F/As on a recent flight I was on around 10 am EST, and I heard back from DL by 2:30 pm that same day. I was pretty surprised - but then again, a short acknowledgement of a compliment doesn't take much in the way of resources, so maybe that's why it was so quick - but that was definitely the fastest I'd ever received a response (I'm also a DM, which I'm sure is a factor).