The number of hours an employee must work to be considered full-time and eligible for employee benefits such as health insurance, paid time off (including vacation leave and sick leave), and retirement accounts (401k and related employer-contributions) is generally in the sole discretion of the employer. This is because an employer is not obligated to provide these employee benefits or fringe benefits to any employee, whether full-time or part-time.
For example, the Fair Labor Standards Act (FLSA) does not define full-time employment or part-time employment. And whether an employee is considered full-time or part-time does not change the application of the FLSA—or the application of the Service Contract Act (41 US.C. §351) or the Davis-Bacon Act wage and fringe benefit requirements (40 U.S.C. §3141).