3 years with an off the shelf engine is probably about the earliest you can expect a new studio to put out a bigger game. Depends on the scope of the game and how much trouble they have in hiring as well.
Even if you're using UE or Unity, there's so much you have to build up in terms of infrastructure. Right now this Stadia team is mostly hiring Engineering/Design/Production leads. Say that goes fast and they have a small team by the end of April. They can then spend May building up some basic pipelines and doing things like concept/prototyping to figure out what kind of game they're even gonna make. Maybe they strike gold and move into pre-production in June. They've still got to hire artists and animators and more designers and more QA, or hire outsource studios to handle that which is quicker but still takes time. Optimistically they move into production early fall and go from there.
The easiest comparison to this situation is Amazon, but Amazon went and at least acquired one existing game studio to try and jump start their process, and that studio STILL hasn't shipped anything. That was SIX years ago (although it is slated to come out in May and they made the worst decision a new studio can make, they chose MMO).
Any time a large corporate entity tries to get into game development there tends to be a long curve of growing pains. Also when you pull people who have spent years at big, established places like EA, Activision, Ubisoft etc and put them on teams of like 10 people where nothing is ready, there's a bit of culture shock for them. None of this is insurmountable, but it generally isn't immediate.
