Judging when to buy new sails is a lot like judging how long you can keep seafood on ice before eating or tossing it. There is a point where they are fresh and there is no doubt, and there is a point where they are rotten and there is no doubt, and then there is a world of opinion in between.
Every time you use a sail, you take a little of it's life and efficiency away. If you are a racer, and performance means more than savings, you would replace sails frequently. The more hard core a racer you are, the more frequently. Top one design boats use a new set of sails for one regatta, and then replace them. For those racers, 3 days is the useful lifespan of a new set of sails. At the other extreme, I know of a few boats where I sail that are flying 30 year old sails with enough stains and patches to make a hobo blush.
Most of us are somewhere in between those two extremes, and I would encourage everyone to consider that none of us has exclusive hold to the right opinion here.