Short Answer: After 40 work hours in a workweek at 1.5 times the regular rate.
Full Answer: The Fair Labor Standards Act (FLSA) requires overtime pay for covered, nonexempt employees who work more than 40 hours in a workweek. The rate is 1.5 times the employee's regular hourly rate. Some exceptions apply to police officers, firefighters, and hospital and nursing home employees, who may have different thresholds under special rules. Additionally, some states have overtime laws, such as daily overtime requirements (e.g., California). A higher standard must determine overtime pay when federal and state laws apply.
General Federal Standard: For hours exceeding 40 per workweek, overtime is due according to Section 7(a) of the FLSA.
Link to a legislative resource from trusted sources: U.S. Department of Labor - Overtime Pay