Preoperative antibiotics are typically administered 30-60 minutes before incision to ensure adequate tissue levels of the antibiotic at the time of the surgical incision. This timing maximizes the drug concentration in the serum and tissues, reducing the risk of surgical site infections. Administering antibiotics too early or after incision reduces their effectiveness because therapeutic levels may not be present when bacterial contamination occurs. Proper timing is crucial for optimal prophylaxis and infection prevention during surgery.

Reference: Surgical Infection, Charles N. Cornell, 3rd Edition.