All fuels 'go bad' at some point. Mostly from the heavier hydrocarbons separating out and forming a thicker liquid that gumms up the works. If you've ever seen a varnished-up lawnmower carbeurator, that's what caused it. This also effectively lowers the octane, making it less suitable for engines. At the same time, the 'good' hydrocarbons are vaporizing out if its not airtight.

The rule of thumb for regular gasoline in ~1 year to be safe. Probably rather longer, but a year is easy to remember.

Reformulated gas, especially with ethanol instead of MTBE, can have a shorter shelf life. Entrained water (there is always some) will cause the ethanol to preferentially separate with the water, and not remain mixed with the gas. Most gas is well under 1% water, but levels of 0.5% will cause this.

Gas stabilizer additives will work, but even they only do the trick for awhile. If you're not using any gas engine for more than a few months, drain the tank.

Diesel will normally last longer, but it is possible to skunk up diesel- an example is in sailboats where the engine may not be used all that much and the fuel can sit for several seasons.