I had this discussion with my oncologist. Five years was the first timeframe prescribed as that was the timeframe of the initial trials. As trials have continued, going longer as the years have gone by, so have the positive results continued.
When I first met my oncologist in December she said I'd be on it for 5 - 7 years. When I saw her for the actual prescription in May, she said several clinical trials had recently released their latest findings and that they'd indicated the 10 year timeframe was showing positive benefits. So she recommended I take it for 10 years.
I asked if 12 and 15 years were likely and she said maybe.
She put me on Letrozole for no other reason than because she alternates between that and Anastrozole as each patient comes through her door!