From: The Real Truth | #006
It depends. Tooie should take longer, there's a lot more backtracking and it's just generally not as fun.
Beyond Good and Evil is somewhere around 8-10 hours I think. Maybe shorter, I forget.
The worlds are also a lot more expansive. Once you get to Terrydactyland, it ramps up to a level that dwarfs anything you've dealt with before. The 5th-7th worlds rival if not completely trump the size of any world you'll find in Mario 64. The last world is huge, but most of it is open space. It starts really daunting and wide, but once you really learn how it works and get your bearings it goes down a little too easily.
Backtracking is essential because unlike the first game the worlds are far more connected. You probably will end up spending at least 10% of your time backtracking to 100% it, if not more.
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Never underestimate the power of stupidity in numbers. ~War13104
Never stoop to the level of idiots. They will drag you down and beat you with experience.