It really depends @ScottHanks. If your tires safely hold air then you can keep using them, I've had a pair of gatorskins that lasted me about 3 years!!
When you start to get punctures or tears in the rubber, you have to be careful. This can actually mess with how the tube is seated in the tire itself. To avoid more tearing after a puncture, make sure whatever made the puncture is out of the tire before putting on a new tube. Also periodically check to make sure your tire is free of glass chips, wood, or asphalt is not embedded in the tire itself.
As for tears in the rubber, as long as it doesn't compromise the tube inside they should be okay. If the tears are all the way through the rubber and are larger then just a small puncture then you may want to think about replacement.
Different tires will last different amounts of time. Unfortunately the more flat protection a tire has usually means the more it weighs. You can find some reasonable tires with flat protection that will last you years.