How long is too long to store avgas? You pump fresh 100LL from a known source (FBO, etc) into a steel (or poly) 55 gallon drum and close it tight. The drum sits outside, either in the sun, or in the shade (could be a cold building) in such a way that water can not infiltrate through the bungs. The drum remains sealed, you do not open it. You do not add any additives, extenders, or isopropyl alcohol.
How long would you let that gas sit before you would not use it for your aircraft? Would that change by the engine you operate in your aircraft? Some engines were designed for 80-87. Some have mods for Mo-gas. There may be other factors to consider.
Would that change by the quantity stored; 5 gallon steel jerry jug, 5 gallon poly jug, 15-30 gallon poly drums, etc?