How many hours does it take to change the brakes on a 1993 kenworth?

As long as it takes. You didn't specify a model (remember, Kenworth makes Class 5 through Class 8 trucks), you didn't specify a brake system (S-cam air brakes, hydraulic brakes, hydraulic disc brakes, air disc brakes, air wedge brakes, or air piston brakes), and we don't know your level of technical experience.