I have a fast friend (28 min 10k) and he made me feel like absolute shlt about my running. "Why bother running if all you can is 17-18 mins 5k?" "Anything more than a 15 min 5k and you're just a hobby jogger" "Why even run?" And it made me stop running for a little bit, cause I thought he had a good point. Why go out there and struggle and put in the miles if all I can run is 17:59? Then I realized he's a dip. Who cares? I wasn't running to impress him or anyone else, I was running cause it made me feel good. I was running to see if I could improve my own times. I never claimed I was a fast runner, I realize where I am on the pecking order, but it's fun.
Having said that: if running stops being fun, I'll stop and find something else: running trails, cycling, swimming, even just walking. So for me personally, it won't be a "time" that stops me from running, but rather once the enjoyment goes. If it starts to hurt to run, I'm not going to put myself through pain for this sport, I'll find something new.