Can journals decide to refuse previously approved publications conditional to minor revisions? I assisted a professor in writing a paper and it got accepted in a journal with minor reviews. I did the modifications I was asked to address and have been waiting on the professor the rest of the time. I've done friendly reminders once or twice to ask what's going on with the paper but decided to drop the ball as I know they are quite busy. This publication is also obviously more important for me as I don't have anything out there yet, so I'm not so surprised it's probably sitting on a hard drive somewhere. We have a great relationship and there will be opportunities for more publications later on. Still, it was a lot of work for me so I'm keeping my hopes up and the topic can change quickly.

If we do end up submitting the revised paper, can the journal turn it down at a later date on the basis that the delays have been too long? Do journals usually have explicit rules with regards to delays for re-submitting after reviews?
Yes, most journals require revisions to be submitted within a certain time limit as specified in the editors decision letter, typically 3-6 months. Failing that, the paper will be considered withdrawn and any re-submission treated like a new submission. However it is also possible to ask for an extension of the revision deadline, if one can provide good arguments.

Of course, I don't know the details of your case, but you should be able to find out from the journal website. 

I also strongly advise you to contact your co-author (preferably in person rather than via email) and enquire about the state of the manuscript and their plans for resubmission.