i think there is an acceptable time frame for an author to not respond to comments. like say the fic was written months ago and you're commenting just now. maybe the writer doesn't even use that account (it's a sock or something) anymore. if the writer is still writing fics normally and doesn't respond to an old fic yet responds to new comments on old fics then it's a bit sad.
i'm only really offended if i've asked the writer a question or soemthing about their fic and they don't get back to me otherwise i'm sure they've read my comment and were happy