How long should I insist?

This is an answer that is based on my opinion and not necessarily fact.

It really does depend on what the topic/problem is. In general I would say that it is best to keep trying, but if you are able to identify a part of the topic/problem that is causing you problems, I would say that you just go ask someone. And when you ask someone, you can always try to ask for hints instead of complete answers.

As an example. If I am reading a paper, and there is a part of the paper that assumes that you know something about for example measure theory, and I don't know much about that, I would definitely just go ask someone for help. Maybe just ask someone for a crash course in say measure theory.

However, if you are working on solving an equation where it is just the arithmetic that is causing you problems, then I probably would just keep trying.

What are the strategies to "keep trying"? You might simply take a break from the problem for a while and come back later. If the problem is simple arithmetic, then you could also try to step back and try to for example write down all the rules/methods of arithmetic that you can think off. Then go over each rule/method and see if applying that in some way might help.