It certainly depends, but in an effort to help out a newbie, I'll post some of my personal thoughts below.
The answers will be, at best, a rough idea. Your mileage will vary, and your experience will eventually be much more useful than anything you read here
Originally Posted by trisstan87
What is the best material for blue-water sails? Canvas or nylon or something else?
Some sails are used all the time, some are only used under certain conditions. The main, for instance, will likely be dacron or similar, while your spinnaker will be probably nylon. Check out sailrite.com for more information on sail material. You won't see real 'canvas' very often.
What is the average life span on well maintained sails?
Your climate and choice of sailing weather will heavily affect this. If you don't stow your sails wet, and if you clean them when something bad gets on them (chemicals, salt buildup, deck cleaners, etc) you can reasonably figure something between 5 and 10 years. If you are a racer, you'll likely replace them every season or two.
At what wind speed does a stout sail face the real threat of tearing?
Your ability to properly trim a sail will have more to do with tearing than ripping. My first rip was because I poorly tied a reef into a main. The wind was a secondary cause. You can rip a sail in 10 knots of wind, especially if it's worn or you have a high-chafe area that you haven't addressed. Here's the important part: sails don't tear because of wind.. they tear because of use. If they are used enough, they'll tear if you sneeze on them.
What sail is the most likely to tear on a boat, I.E. worst track record (the sail that runs out of life first, again maybe the main or the job or another?)
The one you use the most. That's gong to depend on your sailing style. The spinnaker is pretty likely to tear, though, as is the main.
Is it recommended that you keep a spare sail for everything? I.E. Mainstay, jib, ect.
Thats going to depend on your sailing style. If you plan to be away from civilization for a while, you better have enough experience to know which sails to double up on. Thing is, most folks carry multiple headsails, for instance, each a different size than the others. So maybe you tear the #1, and have to sail on the #2 until you get it repaired. On the other hand, many people carry a second spinnaker and/or main, or different types of spinnaker. When speaking of any types of spares - whether sails or engine parts or vhf radios - it depends largely on your storage space and type of sailing. if you are an inland day sailor, there's certainly no reason to carry seven extra sails.
Here's the other piece of advice I might offer... when you are closer to heading offshore, you'll know these answers on your own. So take the above and mull it over so you can sleep at night.. but when it comes time to make the decision yourself, you'll have enough experience that you won't have to listen to a bunch of goobers on the internet