Coming from a Dtoid interview with Ashraf Ismail, director on Assassin's Creed IV...

"one guy in the office actually did it two weeks ago for the first time. He got a 100% sync, which was mind blowing. This was one of our testers, so imagine he's a tester who knows the game really well. He's been working on the game for a year and a half. From zero to a hundred on one build [of the game], it took him 48/49 hours. The thing is, he knew what he was doing is the crazy part."

Ashraf estimates that it could take up to 80 hours for a regular player to 100% the game. For those wondering, the core storyline will take about roughly 20 hours to complete.