That means you can have erections whenever you want or need them. National Library of Medicine. It works best when taken one hour before sex. The as-needed dose is usually 10 mg but can range from 5—20 mg. Which Works Faster?Viagra takes around 30 minutes to become effective and lasts about 4 hours. Cialis lasts much longer — up to 36 hours in some cases. Stendra can start doing its thing in as little . The vagina and clitoris also contain PDE5. So, if a female takes Viagra, the drug may increase blood flow to these areas. However, female genitals contain less . Summary. Cialis and Viagra are both pills for treating erectile dysfunction. They are PDE5 inhibitors that allow for increased blood flow to the penis, leading to more .