I agree that it would be a nice addition to the Constants within the main menu. If you want to change it now, the file is variables.gd (in Strive>files>scripts).
Change var growuptimechild, growuptimeteen, growuptimeadult to whatever amount of days you want.
It's a little game-breaking because of the unlimited amount of slaves and money you can make quickly. But good for testing. I like having a high pregnancy length and a low maturing length.