how long between receiving and R&R and resubmitting?

[pro-cast]

I don't know if this is the appropriate forum, but I was curious about people's opinions on this:
How long is too long between receiving decision letter and resubmitting an article for review?
I had a bunch of chapters from my dissertation out for review during 2012, and then spent most of my time focusing on the  2 that were closest to completion when I got the R&Rs back. Now these 2 have been accepted, but there is a third one that has been on R&R limbo since late 2012. At this point I am wondering if it is worthwhile to start working on it as a resubmission to the place that gave it an R&R, or whether it has been so long that by now I should submit it again as a new submission. Does anyone have any experience on this?

[thoughts]

I would contact the editor. But generally, if they don't give you a deadline, I don't think there is one.

Good luck