This grip is 11 inches wide, and price is $0.30 PER INCH LENGTH. A Typical longboard is about 36 inches long. To find out how much you need, measure your deck carefully following its curved length. Dropped tub decks or camber/rocker will increase the length needed (do not just take a straight nose to tail measurement), maybe add 1 or 2 inches for extra clearance.

This is common generic black griptape.