Go by how you feel.
There are simply too many variables involved to lay down set-in-stone rules for everyone. For instance, everyone is different as far as recovery ability (which is mostly genetic and unchangable), diet, rest habits, training intensity, training volume, training frequency, anabolic drug usage, etc.
Therefore, the best indicator of how you should train is you...imagine that.
However, that being said, I still don't see how anyone (regardless of the aforementioned variables) could honestly train large bodyparts like legs or back more than once per 5-day rotation for an extended amount of time.
Personally, I find that the only musclegroups I can train more than once every 7 days are abs and calves. I have fairly good genetics and my diet is solid but I'm drug free.