Yes, this one is a hard and vague one- in your experience, APPROXIMATELLY what would be the difference in development time when making 2D (2.5D) "point and click adventure" game (like Syberia 1 or 2, broken age etc.) VS. "point and click" 3D adventure like tell tale series or- better- Life Is Strange?

Both would have realistic stylization created in 3D software (3ds max mainly probably) no hand drawn 2D graphics.

2.5D means pre-rendered backgrounds with simple invisible 3D geometry for navigation of real time 3D characters...

Lets say the team is experienced.

Lets say you have some tell tale game like the Game of thrones series, or even better "Life is Strange" game (full 3D "point and click adventure"), lets say the development time of "Life is Strange" is 100% time. What would be the comparable development time, if the studio behind Life is Strange decided in the begging to make Life is Strange as a "standard" 2.5D game (like Syberia 1,2 ; Broken age...)

50% of the time? 75% of the time?

What do you think, give me some RAW ROUGH estimate please...

(engine would be pre made- unity or unreal)

This is really a how long is a piece of string. That said if you only have to worry about one controlled view of each area instead of ensuring that it looks good no matter where you stand and look so as long as your artists and modelers are good it would be less time on the 2.5d fixed view. How much time depends on too many factors to not pick a random value out of thin air tbh

Changing the style of a game does not make the development or production faster, what you save in one place you will lose in a other place.

Its like asking if it's faster to paint a realistic human head or rendering a realistic head; it depends on the artist.

Just look at 2D AAA games and 3D AAA games, they take roughly the same amount of time. Rayman had as much problems in it's development as any 3D game and the story and development doesn't care about the art style.

Quality and size of the game are the largest factors in speed; not style.

You can test this yourself, just model a simple 3D model for a game and the same model, from scratch, for a 3D render and note the time it takes to make it game ready for both. This will give you a rough estimate of how much time you will save in total.