I've googled it which didn't answer my question at all - I've seen 1 and 2 years once opened and I've seen posts on other forums with claims of using the same VG for several years (4 or 5 if I remember correctly).
Last December I bought 2) 500 mls of PG and 1 Gallon and I'm still in them, they both seem fine. What are some signs that either has gone bad?
Bad taste or smell. Try some for future reference. PG will be slightly sweet and bitter, VG will be very sweet, neither will have much of a smell. Both should be completely clear, discoloration is a sure sign. And VG will get less viscous if it absorbs water.
I have PG and VG that is going on 5 years old. I keep them in canning 1/2gallon jars vacuum sealed. They are in a cool dark place inside a plastic tub. I vacuum seal them with a food saver canning jars attachment. The only thing I have had is if it collects water it can taste sour, be too watery. VG will lose it's sweetness. PG will taste like stale water. VG can grow mold with water collected. It can also turn cloudy. I get rid of it if that happens. PG is used in many applications including but not limited to as a refrigerant in HVAC chillers. It hardly needs changing, but in food it does work like a preservative but not as good as salt or sugar. Just keep each in air tight containers so it can't take the water out of the air and spoil. The best advice I can give you is that take a sample of each and keep it in a harsh environment and open to the air and observe what happens over time. That should give you an idea of how they will spoil.
I forgot where i read it ill look for that post but it said something along the lines that vg absorbs moisture and can hold onto to “odors” in that moisture im sure someone will correct me if im wrong but you should be fine as long as you keep them closed and out of sunlight
Both PG and VG are extremely hygroscopic and will absorb moisture like crazy. PG is used industrially for that purpose in many applications. So anything dissolved in that moisture, like things with strange odors or tastes, can be miscible with PG or VG as well, and will help speed along the aging process of those solvents if they aren’t stored properly
The PG is in those bottles with a twist top so I imagine they are good but I keep the VG I'm using I keep in a large bottle with a push cover over the nozzle, not exactly air tight. The bulk of it is in the original plastic gallon. I do keep everything in a cupboard or closet away from heat.
If it's fine, it's fine.
If your mixes don't taste the same, start narrowing down the culprit.
I’m using VG and PG that’s 3+ years old with no issues. YMMV... No changes in anything as best as I can tell.
I keep them in a small hotel like fridge. No problems after same gallons kept for 2 years of large quantity of VG and PG. I have had to toss flavors. Mainly highly ripe fruit flavors and highly sweet or robust smell/taste flavors. Mellow flavors seem to last longer.