A multitude of opinions
You will find all sorts of answers to this question if you search the archives of this site. You can buy a tension guage that will tell you to the pound how "tight" your shrouds and stays are. Or you can do what most seem to do and go by feel. Your stays and shrouds should not flap around under any conditions. What I like to do is hand tighten the shrouds, and go out sailing with a good wind. I put the boat on one tack, and make sure that the lee shrouds are tight enough so that they are still a little taught. I then put the boat on the opposite tack and do the same for the other side. The idea is to have both sides at about the same tension. For the fore and backstays, I attach the forestay/roller furler, and hand tighten. I then tighten the backstay enough so that the forestay/foil doesn't flap around. Not scientific, but it seems to work.