There are a number of ways to waste your time. Because of this it makes sense that "waste" in this context is countable.

Good question. We use this format with a few abstract things. For example, we say: "a waste of space" "a waste of resources" "a waste of energy" We usually say these things to express an opinion. "That ugly sofa is a waste of space." I am not a professor, but I would say that "waste" is not countable here.