If the authorization is for CPT - Curricular Practical Training - the school determines within its internal policies how long a student can work for one employer. Typically, authorization is granted by academic terms. Extensions beyond the first term will depend on those institutional policies. The student should be able to learn those policies and communicate that to their employer.
If the employment is during OPT - Optional Practical Training - the student will be able to work only until the expiration date on their EAD - Employment Authorization Document - whether during their basic 12 month OPT period or the STEM Extension. Employment beyond this date will require employer sponsorship for a work visa