when it's time to break up

[sugar and spice]

I agree with the "You just know" sentiment. I mean, yes, there is a period where you're coming off the honeymoon and things just aren't as good as they used to be, and you start to question the relationship -- but if that period lasts for than a couple weeks, you pretty much start to realize that the relationship isn't worth being in anymore. If, after a few weeks, though, you decide that even if you don't like him as much as you used to, but you still like/love him a lot -- enough that he makes you happy more than he frustrates/annoys/upsets you -- then you'll know that the relationship is worth sticking with.