When do you know it's time to leave?

Ive been with my partner for almost 4 years, we bought our first house together a year ago and I thought that meant that our relationship would soon progress to the next level... Over a year later and many talks about the future he keep telling me that it'll happen but I'm tired of waiting. So tired that I can feel myself losing faith in our relationship and our future. No matter how many times we talk about it it always end the same with him telling me what I wanna hear and then nothing until the next talk. I don't know what to do anymore I'm tired of waiting for something I feel like is never going to happen. How do I walk away from a relationship that I actually don't want to walk away from. Is not committing a legitimate reason to leave?