I think 8 hours

More information here

https://www.tripadvisor.com/Attractions-g155023-Activities-Charlottetown_Prince_Edward_Island.html

You should have more than enough time to go Anne of Green Gables if you have an interest in that. There's lots of museums in Charlottetown. Province and or Government House. A couple of breweries. Lots to do, you just have to figure out what peaks your interest. Look at the cruise offerings and that may also help you research your way to a decision

> I am totally unfamiliar with the area.

When I am in a similar situation, I like to use google maps, especially satellite view and street view, to learn about an area. Here is the Charlottetown cruise port marked: https://maps.app.goo.gl/2jFx9dJd67GJRDwo9 You can zoom in and out to see more detail or a larger area, and drop the little orange person icon onto any blue-highlighted spot on the map for a ground level view, like this one of the port with a cruise ship visible behind a building. https://maps.app.goo.gl/qySR1svSTnpEyU4y7