Premature, very low birth weight neonates are at risk for early-onset sepsis and receive ampicillin and gentamicin post-birth. Antimicrobial stewardship supports short-course antibiotics, but how long antibiotic concentrations remain therapeutic post-last dose is unknown. This study used simulations to examine antibiotic exposures in 34,689 neonates. Therapeutic exposure for ampicillin and gentamicin was evaluated relative to the minimum inhibitory concentration for common pathogens. Ampicillin exposure remains therapeutic long after the last dose. Short-course ampicillin provided therapeutic exposures throughout the typical blood culture incubation period.