Generally, you should render your image until you are happy with the result. After some time, there shouldn't be much noise in your rendering, and almost no quality progress should be visible between the subsequent passes. This is usually the moment when rendering can be stopped.
For an explanation of what happens during one pass and the impact of the number of passes on image quality, see: How many passes is enough?
If you can see some visual artifacts during rendering (like noise or single bright pixels) that won't go away, this probably means that there is either some kind of error in your scene or render settings, or what you are experiencing is a bug.
In such cases, you can try optimizing your scene or report the issue to us:
- What are the best render settings?
- How to get rid of excessive noise?
- How to report issues? (3ds Max | C4D)