How long do you have to go to school to be a McDonald cashier?

Depends. Some chains have their own training programs. Some just expect you to come with some experience or you start at the bottom. Some more "erudite" companies expect you to have at least a B.A. or to have attended a culinary arts school. There are universities that have hotel management degrees such as Cornell in NY. So it runs the gamut. It all depends on what your aspirations are in life, and what the particular restaurant requires. A local diner in your home town may just hire you to start washing dishes or busing tables, shoveling snow in winter and taking out the garbage. Everybody has to start someplace.