Cock ring when I cum

There is no HAVE to with cock rings- experiment and do whatever is comfortable! Like Roxy mentioned, some guys can't come with one on, some can, some like to wear them for extended periods of time...

That said, depending on the type/fit of cock ring you wear, it should be removed after 20-30 minutes. Some cock rings are safe for extended wear, and some are not. Pay attention to any tingling/numbness/pain /cold feeling in your penis- those signs merit immediate removal. Don't wear one while sleeping.

Do a little personal research and find out what works best for you!

As everyone says, it depends on YOU.

However, if you're just starting out with one, can I suggest getting an adjustable one? Like with snaps, or the tie that slides on? That way, it's easier to loosen or get off if you need to do so in order to come, and you don't need to be worried about anything.

And yeah; for your first few times, don't keep it on more than 20 minutes...as you get more used to it, you can work upwards towards 30...but never more than that unless it is designed for it.