To qualify for OPT, an F-1 student must study for a minimum of 3 consecutive quarters (12 units each quarter). F-1 students may enroll in less than 12 units in their final quarter (usually 3rd quarter) with approval from an international student adviser.
An F-1 student who completes a quarter with less than 12 units without approval from an international student adviser is in danger of losing F-1 status.
F-1 students who complete 3 quarters of full-time study (12 units for each 3 quarters) can qualify for a vacation quarter in their 4th quarter if the student plans to return for a following quarter. Students cannot take a vacation quarter and apply for Optional Practical Training (OPT) without resuming their studies for the following quarter. Students cannot enroll in courses in the certificate program when taking a vacation quarter. International students interested in taking a vacation quarter should contact the International Student Advisor to obtain appropriate authorization in SEVIS.
You can visit the Maintaining Your Status Page to learn more about F-1 requirements at UCSC Extension.