Same feel as "how long is this going to take to pull?" Well I don't know if part of what you're asking for exists, how clean it is, and if can join the data you're talking about, so anywhere from 5 minutes to never?
That's exactly how you should respond. I've been on the requester for some of these and if my team gave me that as a response I'd just say "let me know what you find out or when you know more."