According to
an attendee of the developer's annual community event in Poland, CDPR is yet to measure the length of the game when all encounters and "smaller" side quests are taken into account, but it confirmed that the main story and major side-quests will take less time to complete than Witcher 3's.
Game length aggregation site
How Long to Beat measures Witcher 3's main campaign as taking an average of 51 hours to complete.
At the same community event, CD Projekt Red reportedly detailed Cyberpunk 2077's side quests, which are said to be more complex than those in its previous title.
Witcher 3's side quest system was reportedly described as "a single line, with a bunch of branches sticking out of it," whereas in comparison CDPR said Cyberpunk 2077's side missions will often evolve into further side quests which in turn impact other quests, including the main story.