[pcm81]

I am not talking about use in ISO certified lab, just stuff at home type of  thing.
How to determine "real" shelf life of a chemical?
Manufacturers usually advertise 2 years or less depending on the product.
Some substances like resin fluxes can go bad due to evaporation of certain components but if stored closed that does not seem to be a big issue, i have a gallon of resin flux i had for 10 years now, still feels and works like the day i bought it, but it stays sealed  all the time... On the other hand silicone rubber activator partially evaporated turning into pink crystals instead of liquid... while the silicone base component of that same kit is just fine last time i checked.

It seems to me there are several factors that can determine shelf life of "properly stored" chemical. I am curious to hear your opinions on magnitude of their impact and time scale.
1. Evaporation. Applicable to a mixture of compounds, may be mitigated by keeping container sealed.
2. Light sensitivity. Just like KMNO4 too much light for too long kills the substance. Mitigation is to use dark bottles and store in dark place.
3. Slow, unintended chemical reactions in a mixture of compounds. Is this even a factor?

Would any of the above even apply to pure compounds?
Lets say i want to store some MEK, or acetone, or Isopropyl alcohol or similar single compound substances. Would they even have a shelf life or are good indefinitely?

[Corribus]

First, I would say that what qualifies as "still good" depends a lot on the intended use of the chemical. "Pure" compounds can still decompose (either by spontaneous degradation, via self-reaction, via reaction with dissolved gasses like oxygen, via reactions with the container material, or via reactions with trace impurities or moisture), and that process will be dependent on both temperature and light exposure. Whether or not the compound is still OK after x amount of years will depend on the compound of course but also what you're using it for. As an antiseptic or cleaning agent, I imagine isopropanol would be good for decades, but if you're using it for some analytical purpose, as a concentration standard, or whatever, then maybe not so much.

[Enthalpy]

Decomposition happens to pure substances too.
Silver nitrate decomposes under light.
Hydrogen peroxide decomposes at room temperature without light, faster if it contains no stabilizer.