I think 20% of the overall package is fair. If they are going to make the parts where you play as murphy legitimate gameplay sections, I assume their going to fill his bits with puzzles and stuff to make it more interesting. I doubt it will/ hope it won't be just as simple as the multiplayer help stuff that that has been shown of in events.
Either way, If we assume around 10 levels per world/ stage then I'm fine with 2 out of the 10 being different to mix up the game play, like the shooter sections did in origins. This is much better then the 40% of the game being murphy sections stated in a previous news story.
Something's coming up the plumbing, poor Luigi's in a bind. Giant turtles are out to get him, Creepy Crabs are right behind. Spiders, flies. Jeepers! Yikes!, They're coming out the pipes. "MARIO! WHERE ARE YOU!!!"