Depends on what sort of boat you have. For those of us with no boat of our own, it takes about an hour from London to Dover by high speed train, another hour to get to the port, and an hour and a quarter to cross the Channel. That's if you can manage to find a boat leaving an hour after the train arrives. Realistically, allow 4 hours.
Depends on the type of boat and where you're starting from:
Depends on where you start
It depends if you are on an airplane, boat, or a car.
one month
There is no canal between London and Southampton.
a long time
It should take about 22 hours.
Oars and oars
6 weeks
It depends where in France and what type of boat. A cruise ship can reach the south of France from Rome in around 10-12 hours if it didn't stop. It would take over 24 to get to the west or north coast of France.
It can't be driven so it takes no time. You need a boat.
About 3-4 months.