Technically, e-liquid does have an expiry date but it doesn’t go bad per-se.
E-liquid, if stored in a cool, dark and dry place, can have the original taste for as long as 2+ years. This is especially true for e-liquids without nicotine. However, we suggest you use the e-liquid within 12 months after the manufacturing date, especially if nicotine is present.
The longer you store the liquid for, especially in hot environments, the thicker and darker the liquid will become and the more the flavor and nicotine strength will degrade. But this does not mean the e-liquid will go bad, it will likely just not taste the same, or will have a new after taste.
Factors such as heat, length of time, flavors present, strength of nicotine, amount of VG & PG, all contribute to how long it will take for the e-juice to change in taste, color and strength.
How would I know that my e-liquid / e-juice has started to degrade or expire
It will change color, or there will be separation of color. If it is just at the start of this process, you can shake the bottle and if it retains the mixed color after settling down for an hour-or-two, the flavour may not have changed much and you can try using it.
Smells different, odd or tastes milder. You can try a bit of juice in with your vaporizer and taste-taste to check if the floavor is considerably different or not. There is no reported harm from trying this flavour that has degraded in taste.
Feels like lower nicotine strength. If an old juice provides you with less nicotine than you expect compared to what is indicated on the bottle, it may be an indicator of an e-juice that has started to degrade. Again, it is likely still good to vape if you are fine with the taste and strength of nicotine.