The short answer is: it depends on your departure and arrival port and the type of boat but just to give you an idea:

Italy to Corsica:
From Livorno to Bastia – 2 hours
From Sardinia to Bonifacio – 1 hour
From Genoa to Bastia – 6 ½ hours

France to Corsica:
From Marseille to Bastia or Ajaccio – 11 to 12 hours.
From Nice to Calvi or Bastia – on the high speed NGV 3 to 4 hours on a regular boat 7 hours
From Toulon to Ajaccio – 6 hours

For the long crossings most people just get a cabin and travel at night.

As you can see, with so many ports of call and ferry operators the possibilities are endless.

This is why we need reader’s comments. Thanks for your feedback!