Numbness from local anesthesia can last anywhere from 30 minutes to 8 hours, depending on the drug used. Local anesthesia for dental procedures varies widely and depends on the preference of your dentist. Numbness from local anesthesia can last anywhere from 30 minutes to 8 hours, depending on the drug used. In brief: Varies. Local anesthesia for dental procedures varies widely and depends on the preference of your dentist. 