Given how little activities there are on most lemmy posts, I usually comment everytime I find a post worth commenting.
I usually decide whether to comment or not based on comment count instead of time. I'll happily comment on weeks or months old post if it only had one or two comments, but skip over hours old post if it already has plenty of comments.
I do this too. Especially weeks/months/years old post that ask IT-related question that I found in the first place from search engine because I was trying to find the answer. I feel the obligation to leave a reply after I found the solution.