It's the client question I hate: "What kind of response should I get to my email campaign?"
The only honest answer is, "It depends." There are just so many variables. OM Strategy raises this question and links to the results of a MailChimp user survey that has some interesting info that might make guesstimates a little less guessy.
The numbers are broken out by vertical (which is handy). Yes, this just MailChimp's users, and shouldn't be taken as some kind of definitive research, but it's interesting.
Particularly the numbers for technology, which reveal an open rate of 23.75%, and a click rate of 23.27%. That's really astonishing - or it's an error. Maybe techies are just really, really good at deciding whether they're interested in something before they open it?