Time to move on from job?

When do you know it's time to move on from a job and find something new? I work for a small and fast growing company. When I started there 4 years ago, I felt valued and like I was involved in helping the company grow. Now we have basically tripled in size and I am one of the top managers, but I feel like I'm being taken advantage of. I do more than anyone else in a higher level position, even more than the owner in many cases. I work 60 hour weeks on salary so no overtime pay, even going in on Saturdays to catch up when nobody else is there just because I can never get any help from the other managers.

I feel like I do so much with no gratitude and the owner and other manager at my level just keep piling stuff on me without thinking about what it is doing to my sanity. I've tried to talk about it and they apologize in the moment but nothing changes. I'm drowning and screaming for help and I'm becoming resentful toward the owner because he doesn't help me how I feel he should even after I've asked for help. I'm considering looking for other work but it's hard after investing so much time into trying to help this company succeed. When do you know it's time to finally cut ties and move on?