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, nor does it affect application of the Service Contract Act or Davis-Bacon and Related Acts wage and fringe benefit requirements.
http://www.dol.gov/general/topic/workhours/full-time
As defined by the United States government, the standard workweek consists of five eight-hour days for a total of 40 hours per week. The average workday also includes a 30-minute lunch; a full-time employee’s pay for this period is up to the discretion of his or her employer. If the employee’s schedule consists of a 4-day work week, the shifts will typically last ten hours.
http://employment.laws.com/full-time
It is legal to have one set of benefits, or none at all, for part-time employees, and another set of benefits for full-time employees, as long as there is equal employment opportunity within the company. Certain benefits have specific rules, however:
http://www.twc.state.tx.us/news/efte/part_time_full_time.html
Part-Time / Full-Time Status. 1  Texas and federal laws leave it up to an employer to define what constitutes full-time and part-time status within a company and to determine the specific schedule of hours.
http://www.twc.state.tx.us/news/efte/part_time_full_time.html
A full-time job refers to a form of employment in which the employee works the full slate of hours defined by his/her hiring employer. Full-time jobs typically come with benefits (such as health insurance, sick-leave and annual leave) that are not commonly offered in temporary, part-time or flexible employment packages.
http://employment.laws.com/full-time
Full-Time vs. Part-Time Employees. It is entirely up to an employer to come up with its own criteria as to how many hours an employee works a day or week or any other factors that make an employee part-time vs. full-time for that particular employer.
http://www.nclabor.com/wh/fact%20sheets/full_part.htm
Traditional Standard for Full-Time Employment. The standard for full-time employment was typically 40 hours a week in the past. However, many employers now consider employees as full-time when they work fewer hours i.e. over 30 hours, 35 hours, or 37.5 hours. Because there are no laws regulating full-time employment for compensation and benefits purposes, the organization determines how many hours per week are considered full-time.
https://www.thebalance.com/how-many-hours-a-week-is-full-time-employment-2063404
If your employer chooses to distinguish full time and part time employment in order to determine eligibility for fringe benefits, the employer is required to pay those fringe benefits in accordance with their written contract or written policy. The answers provided are not meant to be a substitute for legal advice.
http://www.michigan.gov/lara/0,4601,7-154-11407_32352-129394--,00.html
Full-time employment is usually considered between 30-40 hours a week, while part-time employment is usually less than 30 hours a week. Long answer: The answer is not quite as simple as it sounds. Here’s why. The Fair Labor Standards Act (FLSA) has no definition for part-time or full-time employment.
https://justworks.com/blog/part-time-vs-full-time-employees-what-qualifying-hours
Also, it is legal for an employer to switch an employee from full-time to part-time even without the employee's knowledge as long as the employee does not lose wages, including wage benefits, that have already been earned at the time of the changeover.
http://www.nclabor.com/wh/fact%20sheets/full_part.htm
by law what is considered full time
By law, 30 to 40 hours a week is considered full time. By law 30 to 40 hours a week is considered full time.