How often would you change an alternator for a Ford Taurus?

As often as the alternator fails... ;)

The alternator is not a 'consumable' type of part that requires routine maintenance/replacement - like a serpentine belt, brake pads, etc.

Many repair shops and auto parts stores offer inexpensive or free alternator & battery testing.

Be sure to test before replacing!

NOTE: it is a little known fact that a weak or failing battery is a major contributor to premature alternator death. Any battery that is 4 years old is a prime candidate for replacement.

See "Related Questions" below for information on how-to replace Taurus / Sable alternators