For an offshore passage to visit the Dry Tortugas from Key West, I would plan for a minimum of a week. Use this sailing itinerary as a starting point for your planning!
Day 1: Travel day & Sleepaboard
Arrive, get settled in, and stow away your provisions. Familiarize yourself with the boat's systems if you'd like. Crack a beer! Let's get this vacation underway.
Day 2: Head first for Boca Grande Key
Raise your sails for an early am departure for Boca Grande key. If you have time, grab a mooring ball for some snorkeling and a lunch stop at one of the Marine Sanctuary sites. If you are making great time, you can push on to the Marquesas Keys.
Day 3: Arrive at the Dry Tortugas
Sail the rest of the way to the Dry Tortugas and get those fishing lines out. Explore the magic of Fort Jefferson after most of the tourists have left for the day on the ferry.
Day 4: Daytrip to Loggerhead Key inside the Park
Motor over to Loggerhead Key for the day. Check out the sandbars, explore the lighthouse, and consider snorkeling the Windjammer wreck on the west side. Return to Garden Key in the evening (you aren't allowed to stay overnight at Loggerhead).
Day 5: Back east to the Marquesas Keys
Get an early start to start making your way back towards Key West. The Marquesas Keys makes for a good overnight anchorage. Keep your eye out for lost Spanish galleon treasure!
Day 6: Night out on Duval Street
Complete the last leg of your passage and celebrate your return to civilization in Key West. Grab a bite and check out the scene on Duval street.
Day 7: Checkout and travel day
You've done it! Allow enough time to refuel and complete check-out procedures with the charter company back at the marina.