questions on penetration testing: ratings & # days to resolve

Some questions were raised on our practice of penetration testing:

a) what are the various basis the ratings of Critical, High, Med, Low
    are being assigned?  External-facing servers' XSS will get High
    while internal servers (not exposed to public/Internet) XSS will
    get Med?   There's also various types of XSS that warrants
    different types of ratings?
    Curious how the various tools assign these ratings or in some
    cases, it's the human pentester who assigns it?

b) Is there any framework, eg: NIST, CREST or ...  that specifies
     the duration to resolve?