As with may antibiotics, there is not a definite time in which penicillin begins to take effect. The general rule is to continue the recommended dose until all signs of infection have been absent for 24 hours, according to Hesperian Health Guides medical experts.
Most infections are treated with penicillin for 7 to 10 days. If the infection is still present, a different form of penicillin or another antibiotic may be appropriate, states Hesperian Health Guides. Dosage varies by the age and size of the patient; the lesser amount of the recommended dosage is typically given for smaller persons and minor infections, while higher doses are reserved for larger persons or more severe infections.