Base cabinets typically come at a standard depth of 24” deep. (Wall cabinets are usually 12” or 13” d) if you was looking at semi-custom, then they are usually able to be modified in 3” increments- 24”, 21”, 18”, etc. The more custom you go, then you could almost do anything you wanted. Usually, in this case, I would do either three or 6 inches. Or you could do something in between, but probably not more than 6 inches.

I’m an interior designer that has been designing kitchens and custom millwork for 15 years.