When using STA-BIL Fuel Stabilizer you should add 1oz (30mL) for every 2.5 gallons of gasoline. So if you get 10 gallons of gasoline, add 4oz of the stabilizer.
The makers of STA-BIL Fuel Stabilizer claim that it will "keep fuel fresh for up to 12 months, guaranteed."
Once you've opened the container, the shelf life is about 2 years if stored properly (in a cool, dry area, with its cap tightly placed on it).
To tell if it's still good, look at its color; if it's a bright red then it's still good, but if it has turned a dark red, or a brown, then you should discard it and purchase a fresh bottle.
For other tips, check out their Frequently Asked Questions web page.