When do you know it's time to move on?

I'm always telling people (even when they don't ask for my advice) that they should never settle when it comes to romantic relationships. That's not to say that I expect perfection; I'm obviously flawed myself so that would be hypocritical. That said, when do you know that it's time to move on? For people who've experienced a significant break up, did you have a "light bulb moment," or did your discontent emerge and grow gradually (and cumulatively)? I feel like I'm at a crossroads. And for a bit more context, I do indeed love him. But does love truly conquer all? I know my questions and thoughts are all over the place, but I've been sitting on this for a while and could use a bit of advice.