Take more than 1 day, if you want to see Dover properly. It is a beautiful place bustling with life. People are very helpful. With right sources you get taxis in just 5 pounds! Look up the 2 in 1 Kent offers as it will save you a lot of money. Make sure that you go there on a warm, sunny day as you might have chances to see the french coastline from the cliffs. Keep time for the cliffs. With proper preparation and gear, you can enjoy to the fullest!