How long can you leave a 2 year old to cry before you comfort him or her?

I have a two year old son who just doesn't want to go to sleep at night. I let him cry for about ten min, and sometimes he settles down, and other times he doesn't.

I do go to him if he crys for more than ten min. Is that all right? Should I let him continue to cry, or should I go to him sooner?
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