Can grass grow really long

Not really. Depending where you live, your neighbors may disapprove of it, though.

Answer Grass grows to it's natural length if left alone. This will vary depending on the type of grass. The only reason for cutting it is to make your place look tidy, or, like everything else in the garden, cut it down in the Autumn to keep it vigorous for the next year. You wont have a lawn though.