I think through trial results they've found that 5 years seems to be the most efficacious plus 5 years is the magic number for being pronounced cancer free. Though i have read that there seems to be a thought out there that maybe people should be on AIs for longer but that would have to be on a case by case position. Letrozole seems to be the flavour of the month. That and anastrozole have a similar make up but exemestane is steroid based. Becoming a dried up prune? That hasn't happened to me totally...yet!