In the US, the period of a patent is currently 20 years from the initial
application or 17 years from the granting of the patent.  But it can
take some time between the granting of the patent and the time a drug is
approved.  In addition, a particular drug may involve more than one
patent.  Also, patent holders can extend the period of a patent by
various mechanisms.  Finally, patent holders for drugs often invent ways
to discourage companies from manufacturing generics after the patents
have expired.  So I don't think you can generalize, but 10 years from
FDA approval might be a typical value.

Sometime in the next several years, legal generic versions of Viagra
will be available in the US.  But that won't necessarily mean that the
drug will dome down much in price.  That may take quite a bit longer.

Of course, generic, illegal versions of Viagra are freely available on
the web, if you are willing to chance their being safe and effective.