Writers: How to know when it's time to be done?

How does one decide it’s time to end a story that’s in the works?

I’m writing a novel and, even though I know it’s kind of a bad idea, I haven’t had it all planned out from the start. I haven’t come up with how exactly I want it to end.

Yesterday I finished a chapter and realized everything was wrapped up fairly neatly. Now I’m second guessing as to whether I should continue. I have more ideas, but at this point I might be continuing just for the sake of continuing because I’m enjoying the writing process. I don’t want to drag it out past its logical end, but I also don’t feel totally satisfied right now for some reason.

I know it’s a very subjective question. But how do you know when to stop?