There are 200 levels, most of which are not too tough to finish. The final levels are hard, so difficult I've had issues where people have claimed that they are impossible to solve before, hence my warnings. But you also get the option to skip 10 levels and you can get to one of the game's endings by finishing 80 levels (as the zones branch into multiple paths after the first tutorial zone).
I'd say that the average player will be able to get to at least one of the four game endings, but it would take a real masochist to solve all 200 levels.