Modern sails seem to last a very long time without ‘blowing out’. Replacing them really becomes a matter of personal taste when you can longer put up with the performance. If you are happy to potter about and sail everywhere on a beam reach, you could probably get away with never replacing your sails 🙂
It cost me about ÂŁ100 to have two lines of reefs put into a mainsail… so I wouldn’t spend that money on a truly tired old bag of a sail, but maybe save up and get a new one with reefs included.