Say I have an issue, and I find a helpful question and answer that was posted in 2015. There being back and forth on the answer is it proper to join the comments asking for followup today in 2017 or is it best to open a new question?
I wouldn't say there's a hard limit, but it's more about how involved your follow up is and how frequently those members contribute. In general, if your follow up can't be answered in one line, ask a new question. Make sure you link to the old one if it helps provide context, otherwise it might get mistaken as a duplicate.