Diablo V may have been in development for about three years, and Blizzard, according to one employee's profile, may have started working on the game shortly after the release of Diablo IV.
The new installment of the series will tell the story of events taking place 100 years after Diablo IV. The heroes lost, Sanctuary fell, and Diablo triumphed. The release is scheduled for spring 2029.
Information on a Blizzard employee's LinkedIn indicates that work related to Diablo V may have started as early as September 2023 or no later than March 2024. If the data indeed refers to the game, its development has been ongoing for approximately two and a half to three years.
This does not mean that Diablo V has been in full production all this time: the early stage could have included concept, technology creation, and prototypes. Blizzard officially confirms only the game's development and a spring 2029 release.