Its effects often last for 4 to 5 hours. Ask your pharmacist or doctor for a copy of the manufacturer’s information for the patient. What are other possible causes? Because appointments can be brief and there’s often a lot of ground to cover, it’s a good idea to be well prepared. Vardenafil usually works for 4 to 5 hours. When choosing one, tell your doctor or other health care professional what your preferences are, such as cost, ease of use, how long the drug’s effects last and side effects. The best time to take sildenafil is about 1 hour before sexual activity, but you can take the medication any time from 4 hours to 30 minutes before sexual activity.Generic Viagra (sildenafil) is available as a , , , and milligram (mg) pill and . Below are details about Viagra dosages commonly used to treat erectile dysfunction . Follow all directions on your prescription label. Do not take this medicine in larger or .