2 years. Take it for 2 years until you have a follow up DEXA scan done to evaluate the improvement in your bone density. It is debatable on how long you can take biphosphonates (the class Fosamax (alendronate) belongs to). Some say you may take it indefinitely, while other recommend taking it for 4-5 years and then take a "holiday" for a year or two. As I mentioned, take it for 2 years and see what happens.