According to Edge Magazine and Reddit, Destiny 2 will feature over 50 hours of content when it’s released on September 6 on Xbox One and Playstation 4 and October 24 on PC.
Edge Magazine states..
“In total there are over 80 missions and activities in Destiny 2, and each is substantial—in length, challenge, story, and reward.”
It’s also been said that the story was even longer with Activision telling the magazine that play testers reached the end of the game at around 55 hours with more PvE content left to play. The story has since been cut down, but it’s very likely that this additional content could be later added as part of a major patch or as an Expansion.