I'm guessing it's about 250 nm if you go up the Bay to the C&D Canal and then down to the Dover area. You didn't mention what boat you have, or what you expect your average speed to be. Divide 250 by your average speed in knots and you'll have a time estimate.
If you plan to sail, you'll need to take into account the fact that the Potomac
River is fairly narrow and has many twists and turns until you get past the Rt 301 bridge, so expect to do a bit of motoring. In the summer, the prevailing winds in the Chesapeake tend to be from the SE or S, so heading north up the Bay should be a good route.
Where will you come ashore near Dover? Can you get up the Leipsic River, or go into Bowers Beach? There's no deep water
access to Dover proper that I know of, so you'll need to arrange for a ride to town.