I answer based on interest and/or time. Sometimes there's a PM that I know I will need a little extra time to answer and so it sits there until I have time to give it the attention it needs. I typically answer PMs from people I know before I get to the out-of-the-blue ones. Now, having said all that, I rarely find myself with more than a couple to get through, so it's a pretty quick procedure.
Certainly not as many as I would like. I'm sure there are many more of you with delightfully depraved thoughts and I really think it would be good for you to share them with me. X