How long is reasonable to wait for reply from an editor?
There are good reasons to expect communication with an editor to be slower than many other e-mail exchanges. Editors are busy, especially since editing may be a minor but time consuming part of their job. And responding to questions from authors often involves contacting referees, who may in turn be unresponsive.

I'd say that it's reasonable to wait a couple of weeks for a response to a timely request for a status update (two months after a revision strikes me as a bit early---I'd expect it to take longer---but not unreasonably early to ask for a status update). If it gets lost (for instance, the editor is waiting for a response from the referee, and hasn't remembered to tell you that's what's happening), a polite reminder and a few days to another week or so of waiting might be appropriate. 

After that, though, you can rightly begin to worry that you aren't hearing back. If there are other editors, especially a chief editor, you can contact, I'd start there. I'd be very reluctant to unilaterally withdraw the paper, since that might cause complications if the editor returns and feels you resubmitted while it was still under consideration.  Before resorting to that, I'd try to talk to one or two senior people in the field for advice about the specific paper.

(Just to emphasize, I'm talking about communication with an editor about the status of a paper. Of course the refereeing process as a whole can take a year or more.)