How Long Do Viagra’s Effects Last?

Question:

After diagnosing me with mild to moderate erectile dysfunction recently, my doctor prescribed Viagra — 50-milligram tablets — to be taken 30 to 60 minutes before sex. I have heard varying accounts of how long Viagra lasts and would like to have an expert opinion on this question.

And does the drug’s duration of effects count from the point when it begins to take effect or from the moment the drug itself is taken?
Barry H., Helena, MT