How do you know when it's just a rough patch vs time to end a romantic relationship?

How do you know when it's just a rough patch vs time to end a romantic relationship? I know it's quite personal and varies by relationship, so I'm more interested in folks' experiences than in a general rule or standard. (Although if you do have a rule of thumb to share, would love to hear it too.)

Note: I'm not speaking of abusive situations, but of relationships with ordinary troubles.