The length of time a product will last unopened varies and is influenced by temperature, so read the label to find out how fresh it needs to be. Once opened, many products degrade more rapidly. Since disposal of these items can present environmental problems, it is both economical and prudent to buy just what you need for the job. The same holds with many garden chemicals. I try to buy only what I will use in a season.
Gasoline also degrades with time.
Otherwise, the gas will degrade, leaving deposits, called varnish, on parts and clogging small orifices, called jets, in the carburetor. Clogged carburetor jets result in a lot of pulls with the rope to no avail. If a potato chip doesn't have the proper crunch or your favorite herb or spice lacks the usual intense aroma and seasoning power, you know it is stale. All you need is a glass of water. If the egg lies on its side under water, it is fresh.