Rejections should teach you something.
I think of it like the sculptor: Rejections should be like the process of removing all the material that is not part of what you are sculpting. Each rejection should chisel another chunk off.
If it seems like it's been a while since you learned something from a rejection it's time to review.
Even after refining to the point of having a finished sculpture, you still need to consider the possibility that you've got something good, but simply haven't pitched to the right person yet. Look for comments like, 'that's a really compelling|interesting|novel|worthy idea, but it's not what we get involved in'. In those cases, ask for referrals to people they think might be interested.
The point to cease then, would be when you're not learning anything new, and you're not getting references to someone who might be interested.
Each of the examples you listed would have a different protocol. If you get a no when asking for a raise, or pitching investors, you should be able to get an explanation for why. But if you get rejected asking for a date, you'll likely not get a reason you can use. So some intuition is always going to be required.