Compensation of salary pay and work hours.

The answer to this question depends on your "classification" (the legal term of art) and whether you are properly classified as an "exempt" (i.e., not subject to the provisions of the federal Fair Labor Standards Act) employee. The fact that your employer chooses to pay you a fixed salary does not dispose of the issue of whether you are exempt or non-exempt; rather, what matters is your duties. It is not possible to tell from your inquiry the nature of your duties and whether you fall into one of the applicable exemptions. Assuming you are, in fact, exempt, then you are paid by the job and not by the hour, and your negotiated salary covers all hours necessary to perform your tasks as determined by your employer.