It really depends on the game. For a game within a franchise that has a strong release schedule like FIFA, preproduction can be as short as 4 months as the developers figure out what they have the time to do. For other games, it can be longer - the dev team can spend up to a year in preproduction figuring out the core gameplay for most games, with more time needed for games with really massive scope (e.g. MMOGs). AAA dev really tries not to spend that much time in preproduction because the plan should already mostly be in place by the time we begin - we know what kind of game we’re making and we have a solid idea for what kind of gameplay we’re trying to build out. The goal of preproduction is to build out to the vertical slice of gameplay, a playable example of all of the game’s features and systems working together.
It’s worth repeating - the vast majority of AAA games with 5+ years of dev time aren’t usually being fully built from start to finish. Most games with long dev cycles get started, things don’t evaluate well, the progress gets thrown out, and development gets restarted. Only a handful of game franchises are exceptions to this - Elder scrolls, GTA, whatever Riot or Valve are doing, etc. Most studios just don’t have the financial freedom to burn money for multiple years without anything to show for it.
As for your second question, it also depends. After the post-launch vacation time, the dev team usually gets split up. Some of the dev team members usually continue on with the launched game to work on additional DLC content, cosmetics, and so on. Some of the devs can get moved onto another project that’s already in progress, usually as part of a ramp-up into production. A few of the more senior team members might be tapped to go work on really early development for new game ideas. And some of the dev team will inevitably decide that they’ve finished their last hurrah and tender their resignations to go seek greener pastures elsewhere.
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