Does anyone know just how long you can keep ( and use ) gas with stabil or any other stabilizer? also what would yuo use to clean out a vehicle and tank that hasn't been touched for 5-10 years? Thanks in advance for any suggestions.

Best answer will be found on the label of the stabilizer bottle itself. They vary. Try pulling the tank and having it boiled out at a radiator shop (some will leave it in their tanks over the weekend). If not, commercial cleaners and vinyl sealers are available from suppliers in Hemmings, etc. Rust can be broken loose by installing a larage handful of nuts and bolts in the tank and spending a very tiring hour shaking the tank. Or, put it in the back of your pickup bed and leave it there for a week or so. It will be derusted by then. Clean, seal and there you are.

What about cleaning out a fuel system that has not had the Stabil put in it but has just been "left to die of old age" any suggestion on cleaning that gummed up mess out- short of total teardown?

If the inside of the tank is rusty I would just use a sealer in it to insure that it would not leak and make sure you don't get rust in the fuel system.....
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Grump

Life depends upon where you live, temperature and EPA gas formulation.
In this part of New England I had a mess in my lawn tractor and emergency generator in just one winter. Straight gas will go bad in a few months during the summer.

Another approach to derusting a tank, after the nuts and bolts break off the loose stuff, is to immerse it in a container with a 3:1 or 4:1 water to molasses mix. Takes 2-3 weeks of fermenting and keep the lid closed and away from the house; it gets pretty ripe.
High pressure wash the muck off and and then patch up any leaks. Coat the inside and outside with a good sealer ASAP so rust doesnt form again.
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Carl
Automotive gold is everywhere; you just have to learn how to dig for it