I think it really depends on your reasons for writing. I write fiction because I like making words turn into stories. I'm actually a lot better at original writing than I am at fanfiction, but the basic reason is the same: I enjoy it.
As my main reason for writing is for myself, I stop when I'm either not having fun or RL issues get in the way. Sometimes writing helps me deal with stress--but if it's creating stress, it needs to end.
That being said, original fiction, with intent to publish, is different and something you may have to FORCE yourself to write some days. Fanfiction, in my life, isn't. If I don't have time, I don't have time. If I don't feel any words or ideas coming, I don't. It's not like a research paper with a deadline that means you have to write whether you want to or not. It's purely voluntary. And sometimes you get someone who reads it and likes it. That's always a plus.
As to how you know it's "time"--I think it all depends on the person. Obviously many people have hit a point in LNC where they have no more ideas, desire, time, what have you. They know it's over for the time being, and I respect them for it. Doesn't mean I won't wish they were still writing though!
