How many hours in a day can an employer work you before you can get rest?  

Except in certain specific industries or jobs, where hours are capped for safety reasons (like truck drivers, airline pilots, doctors in certain states), there is no upper limit--an employer can require you to work as many hours as the employer wants, subject only to the following:

1)  If you are an hourly employee, you must be paid for all hours worked.

2) If you are not exempt from overtime (and hourly employees are never exempt, with a very few exceptions, like certain sales staff; that is, they can almost always get overtime), you must be paid overtime when you work more than 40 hours in a week.