According to the developers, a Baldur's Gate 3 campaign can take an unhurried player 75-100 hours. A completionist playthrough, Larian says, would take more than twice that; 200 hours for a comprehensive playthrough of everything the game has to offer.
We'll have to wait until more people beat the game to test the veracity of those claims, and we'll update this page once an unbiased picture emerges. It's probably worth pointing out that playtime refers to a single Character, with your choice of Origin and Class offering content unique to that campaign, so there is plenty of incentive to play other characters, too. The levelling process is also quite slow in comparison to other big-budget RPGs, with a surprisingly low level cap, which is Larian's way of telling you not to rush.