How do you know when it's time to break up?

I'm in my first relationship. November will be two years. Since I met him I knew he wasn't the one I was going to marry, but things went well and I was intrigued to go further being I was never in a relationship before. Well now we're comfortable with each other and obviously friends. I'm not IN LOVE with him, but I love him for loving me.

I've been honest with him multiple times by telling him I don't see myself marrying him/I'm curious to date other people etc.
It's always brushed off like "don't stress about it, we haven't been dating that long"

We've kind of had a very easy relationship. Taking it day by day. Which I liked. But lately I've been getting more curious what else is out there.. I feel like I could be missing out... At the same time- it scares me to completely end what I have now.
I never experienced a break up before. Ugh :(
How do you know when it's time to break up?
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