If you're talking about a diocesan priest, then the 8- or 6-year thing is generally true. It's different, though, if you're talking about joining a religious order.
If you have a Masters in Philosophy, your pre-theology formation might be a little shorter (the possibility exists, but it isn't carved in stone): any other Masters, though, would be irrelevant, and wouldn't be a reason to shorten your pre-theology formation. Remember, though, that 'formation' is more than just academic work, so even if you have an MA in Phiosophy, there are good reasons why you might take the full two years before working on a theology degree.
(And, of course, if you already have an M. Div., you'll still generally spend four years in theology; more than likely, you'll work toward an advanced degree, while you do your theology formation...)