How can we know exactly when criminals are suitable to fit into regular society?

I was thinking about it, and when should criminals be released into society? Is there a time limit? I know that there’s a certain time limit on releasing criminals, but what is it? And how do we know when they are able to be released? I know that all criminals are not the same, and saying, for example, that two weeks is sufficient and good behavior is sufficient isn’t really suitable for everyone.