Bad Gas

Here's Why
Unlike crude oil, gasoline is a highly refined product brewed to a certain chemical composition with very specific characteristics. One characteristic of gas is volatility, a term used to describe how easily and under what conditions the gas vaporizes so it can be efficiently burned in your car's engine. The most highly volatile components in gasoline also tend to evaporate over time. As they do, the remaining fuel's volatility and ability to combust properly degrades. The result is diminished engine performance. Your engine may still start and run, but it probably won't run as well.

A Serious Problem: Oxidation – Varnish / Gum
Hydrocarbons in the gas react with oxygen to produce new compounds that eventually change the chemical composition of the fuel. This leads to gum and varnish deposits in the fuel system.

These deposits and impurities can clog up gas lines and filters, as well the small orifices in a carburetor and the even smaller orifices in a fuel injector. Removing these deposits can be expensive and your vehicle may not run at all or run very poorly until they are removed.

Water Contamination
Condensation can form inside your gas tank and lines from heat cycling when we store our cars in unheated areas, inside or outside. Fuels such as E10 or E85, contain ethanol alcohol which increase water contamination, as ethanol likes to draw moisture out of the surrounding air.

Water, of course, does not work too well as a fuel in an internal combustion engine. It will cause hard starting and rough running until it's purged from the system. It can also contribute to internal rusting of the gas lines and tank.

The resultant scale and small particles can create a true nightmare, sometimes requiring the replacement of the gas lines and tank at considerable expense.

How Long Does it Take for Gas to Go Bad?
That depends on a number of factors. For one, it's hard to know how old the gas you just bought actually is. It may be fresh from the refinery, or it may be a month old already by the time you top off your tank. Some gasoline is mixed with better or more oxidation inhibitors than others.

Want to do?
It's a good rule of thumb to avoid leaving gas in your tank or a storage container for more than a couple of months.

And if You Can't?

To prevent or reduce water contamination:

You can reduce the chances of water contamination by keeping your car's gas tank as close to full as possible, especially if the vehicle is going to be left idle for an extended period.

Or Evaporation of volatile compounds can be limited by making sure the gas cap is secured tightly. For the same reason, be sure all portable gas containers are sealed tightly as well.
Draining the tank and lines will reduce but not eliminate water damage as a dry system can expose the insides of metal fuel lines and your gas tank to air and moisture, leading to accelerate formation of rust.

To prevent or reduce fuel oxidation:

If you know gas will sit in your tank or a storage container for more than a couple months, then it's a wise move to buy some fuel system stabilizer and mix it in with the gasoline. Run the car long enough to be sure the stabilizer has gone through the fuel system.

The stabilizer helps prevent oxidation, the biggie that can turn gas into garbage that gunk's up your system and leads to expensive repair work.

Using fuel system stabilizer for extended storage is preferable to draining the tank and leaving the system dry. Fuel Stabilizer will keep fuel fresh for 12 to 15 months.

If you're going to leave the vehicle parked for and extend period of time then draining the tank may be the only option even with its short comings.