Probationary employment shall not exceed six (6) months from the date the employee started working, unless it is covered by an apprenticeship agreement stipulating a longer period. The services of an employee who has been engaged on a probationary basis may be terminated for a just cause or when he fails to qualify as a regular employee in accordance with reasonable standards made known by the employer to the employee at the time of his engagement. An employee who is allowed to work after a probationary period shall be considered a regular employee.