In a recent interview with French outlet Jeuxactu (via IGN), Lead Cinematic Animator for CD Projekt Red's Cyberpunk 2077 Maciej Pietras revealed that development on Cyberpunk didn't start in earnest until after the release of The Witcher III: Hearts of Stone DLC. The Cyberpunk 2077 development cycle has been rather opaque, but we did have hints about when CD Projekt began working on their new RPG thanks to hidden messages in the game's E3 trailer.
Pietras' interview has a more nuanced timeline, which slightly contradicts the hidden message.
Though Cyberpunk 2077 was formally announced in January 2013 through a brief teaser trailer (following a tease of the game in 2012), it wasn't until after the release of the first Witcher 3 expansion, Hearts of Stone, that work on Cyberpunk 2077 began in earnest. In fact, until The Witcher 3 was finished, it appears full production on Cyberpunk 2077 was pending to an extent.
Pietras mentions that this was around 2014, though Hearts of Stone wasn't released until October 13, 2015. In fact, The Witcher 3 itself wasn't released until May 19, 2015, implying that either the year referenced in the interview wasn't correct, or proper work really did begin prior to the release of The Witcher 3. However, soon after in the interview, Pietras says the team was entirely dedicated to the game from 2014 onward, suggesting a sizable team was already in place while The Witcher 3 was finishing production.
In a hidden message included in the E3 2018 trailer of Cyberpunk 2077, CD Projekt Red revealed that "As soon as we concluded work on Blood and Wine, we were able to go full speed ahead with CP2077's pre-production." Blood and Wine, The Witcher 3's final major piece of DLC, was released on May 31, 2016, suggesting that the full weight of CD Projekt Red's development muscle wasn't behind Cyberpunk 2077 until after this.
While the timeline of exactly when the developer began properly working on Cyberpunk 2077 remains unclear, it's certain CD Projekt Red has been developing this game to some extent since 2012 or earlier. By 2013, a team of around 50 people had reportedly started working on Cyberpunk 2077. Between then and Hearts of Stone, it appears the development team grew, and after Blood and Wine's release in 2016, full development on the title began.
As IGN points out, even Pietras' development timeline is rather unclear and seems contradictory. Unfortunately, given the heavy secrecy of the project, its likely we won't get a proper development timeline for Cyberpunk until after the game ships.