The following info from PDR may help:
For most patients, the recommended dose is 50 mg taken, as needed, approximately 1 hour before sexual activity. However,
VIAGRA may be taken anywhere from 4 hours to 0.5 hour before sexual activity. Based on effectiveness and toleration, the dose may be increased to a maximum recommended dose of 100 mg or decreased to 25 mg.
The maximum recommended dosing frequency is once per day