stiffen sails - how often?
Recutting a sail is something that only a sailmaker should perform. They will look at the sail and see whether or where it needs improvement and add or remove cloth as necessary.
Technology being what it is, most sailmakers can use a photograph of the sail taken from mast head or foot and software can calculate whether a change is necessary. It's important to realize that a sail is not a triangular piece of material that magically catches the wind, it's an airfoil and lift is everything. Shape = lift and most casual sailors have very little idea what the correct shape should be. In short, recutting a sail is not something an amateur should attempt, IMO.
We have a pretty new genoa made by an offshore loft. I thought that the luff had too fine an entry and the whole thing was too flat. Our current sailmaker ran a photo through their software and proposed recutting the luff and wading the back stay. They added a tapered strip of cloth along the luff and the result us a far better shape that is easier to keep in the groove. Amazing what a 1 1/2" piece of material in the right place can do.
Sabre 38 "Victoria"