I say manga is too long once it runs past its original idea phase. With naruto the original phase was clearly to the end of the, was it the water village arc? The first time there out on a mission. There's a clear shift from the focused storytelling that was there prior and the mess that it became after. With bleach it was through the soul society arc, it was so clear that up through soul society everything was planned and then afterwards everything was improvised. Now if you're gag the manga and your premise is mainly character interactions with this setting as the backdrop of give you leeway for stretching it.

I read this initially as 'number of chapters' and selected 'it's never too long' since 200 seemed too few, but 200 volumes is pretty extreme! I would happily read 200 volumes of series like Liar Game, but I doubt authors could maintain quality for that long - if the author runs out of ideas, euthanize it.