How long should I render my image?
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 subsequential passes. This is usually the moment when rendering can be stopped.
How do I set limit for rendering? (3ds Max)
For an explanation on what happens during one pass and the impact of 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.