It's not that old fuel loses the chemical energy needed to power an engine, but the problem for motorcycle use still is twofold (at least). First, the most volatile compounds evaporate fairly quickly, which makes starting more difficult. Second, especially with ethanol blends, a membrane forms inside the pilot jet as the fuel evaporates, which makes starting nearly impossible.
I've probably written about this before somewhere on this site so I won't repeat all that information. Briefly, though, in an open container the size of a carburetor bowl, fully half the gasoline will evaporate in three days. However, to better simulate the actual situation, 18 years ago I started what is perhaps the world's longest-running motorcycle experiment. The initial part of that experiment was to test the efficacy of fuel stabilizers, and the long-term part is to measure what effect, if any, ethanol has on the metal used for carburetor bodies and jets.
Making a long story short, I estimated the open areas in a carburetor through which fuel vapor can escape to be approximately equivalent in area to that of a 5/32" hole. I drilled such holes in the lids of several containers, placed pre-weighed foil cups in those containers holding several types of gasoline with and without ethanol and with and without the maximum recommended dose of stabilizer, and measured the weight loss due to evaporation as a function of time. After 676 days (1.9 years) there was still a small amount of viscous liquid left in all the containers. The important point is that throughout the experiment the stabilizer had no effect on the evaporation rate for any of the fuel blends, i.e. at every point in time the weight loss in all the containers was the same. The volatiles left as quickly in the initial days, and the volume of viscous liquid remaining nearly two years later was the same, whether or not the gasoline had been "stabilized."
Obviously, nothing will evaporate from a sealed container, but since I don't keep containers of gasoline around, I've not done any experiments that apply to them. I realize there are other opinions but, as a result of data from the motorcycle-relevant experiments that are only briefly described above, I don't spend money on fuel stabilizers. If I don't plan to ride a particular motorcycle again for more than a week, I take the time to drain the float bowl. Although I haven't measured the evaporation rate from the small hole in a fuel tank cap, if I won't ride it again for at least a month, I also drain the tank.
