This is a loaded question! The answer every sailor wants to hear is, ‘forever.’ Or at least, ‘a very long time.’ In reality, the answer is more complicated. As a triangle, sails will last for a surprisingly long time. Structurally, sails gradually lose their integrity as the materials and stitching fail under the influence of the sun and use. The second part of the answer has to do with shape. Sails which stretch too much are unable to retain a critical airfoil shape (having a distinct rounded entry and flat, straight exit). This failure in shape costs you in subtle ways. Read more here.