Dacron sails last a long time with proper care. When they start to get patches they are ready to blow out in a gale. It is important not to lose sails altogether because it is always an inopertune time. On larger boats, having several sails to chose from always for one to reduce sail to a smaller size, ie storm jib instead of Genoa.
Canvas sails last if they are never put away wet, like canvas tents. Ultraviolet light is hard on sails. If they are not stowed below, they should be carefully folded in place and a sail cover should be used.