How long does it take from May Penn to the Dunn's River falls?

It depends alot on 2 things, traffic & the driver. Your nearly on totally opposite ends of the island, plus that route takes you through Portmore ( very busy & populated ). But you can count on at least 3 hours. Do not attempt the trip on a Friday or Saturday evening, as trucks break down going over Mt Ra and the trip can take 5-6 hours waiting in traffic !