Part-time employee working full-time hours

You may not be able to demand full-time benefits; that will depend on company policy.  However, you may be entitled to certain pay requirements pursuant to the Fair Labor Standards Act (FLSA) . 

While FSLA does not define full-time employment or part-time employment (again this is a matter generally to be determined by the employer), whether an employee is considered full-time or part-time does not change the application of the FLSA benefit requirements.  For example, for covered, non-exempt employees, the FLSA requires overtime pay to be at least one and a-half times an employee's regular rate of pay after 40 hours of work in a workweek.

Note:  Some states have overtime laws.  In cases where an employee is subject to both the state and federal overtime laws, the employee is entitled to overtime according to the higher standard (i.e., the standard that will provide the higher overtime pay).