29 C.F.R. § 541.700 : Labor — REGULATIONS RELATING TO LABOR — WAGE AND HOUR DIVISION, DEPARTMENT OF LABOR — REGULATIONS — DEFINING AND DELIMITING THE EXEMPTIONS FOR EXECUTIVE, ADMINISTRATIVE, PROFESSIONAL, COMPUTER AND OUTSIDE SALES EMPLOYEES — DEFINITIONS AND MISCELLANEOUS PROVISIONS — Primary duty

(a) To qualify for exemption under this part, an employee's "primary duty" must be the performance of exempt work. The term "primary duty" means the principal, main, major or most important duty that the employee performs. Determination of an employee's primary duty must be based on all the facts in a particular case, with the major emphasis on the character of the employee's job as a whole. Factors to consider when determining the primary duty of an employee include, but are not limited to, the relative importance of the exempt duties as compared with other types of duties; the amount of time spent performing exempt work; the employee's relative freedom from direct supervision; and the relationship between the employee's salary and the wages paid to other employees for the kind of nonexempt work performed by the employee.
(b) The amount of time spent performing exempt work can be a useful guide in determining whether exempt work is the primary duty of an employee. Thus, employees who spend more than 50 percent of their time performing exempt work will generally satisfy the primary duty requirement. Time alone, however, is not the sole test, and nothing in this section requires that exempt employees spend more than 50 percent of their time performing exempt work. Employees who do not spend more than 50 percent of their time performing exempt duties may nonetheless meet the primary duty requirement if the other factors support such a conclusion.
(c) Thus, for example, assistant managers in a retail establishment who perform exempt executive work such as supervising and directing the work of other employees, ordering merchandise, managing the budget and authorizing payment of bills may have management as their primary duty even if the assistant managers spend more than 50 percent of the time performing nonexempt work such as running the cash register. However, if such assistant managers are closely supervised and earn little more than the nonexempt employees, the assistant managers generally would not satisfy the primary duty requirement.

29 C.F.R. §541.700