How long would a salt water fish live in a salt water tank?

This also depends on how good your filtration system is. You can have a huge tank, a crappy filter, and only a few fish, and they will still be stressed and sick from the ammonia and nitrate buildup. They will eventually die. However you could have a relatively small tank, Excellent filtration, and a few smaller fish. So it depends on your filter and how big you tank is.

The General Rule is litre = 1cm of fish, 1 inch of fish = 1 gallon

In general, there is no rule of thumb for saltwater aquarium fish. It really depends on your filtration and the design of your aquascaping. If the fish are not stressed and you're water parameters are in check, then you will be able to keep an almost limitless amount of fish in your saltwater aquarium. Now this doesn't mean put 5 yellow tangs in a 10 gallon aquarium. Use common sense and make sure the fish have enough room to swim.