How do you know when e-liquid has gone bad?Updated a year ago

Vape juice will eventually reach a point where you will no longer want to use it when vaping. This is because it gradually degrades and won't be comparable to when it was fresh from the store. 

Part of why vape juice has a shelf life of 12-24 months is due to this degradation and no longer meeting the manufacturer's quality standards. There will be a noticeable difference in flavour with e-liquid that has been stored for months. You might not even be able to initially tell if your vape juice has aged, as it's not as though the vape juice will have a drastic difference.  

Once the e-liquid bottle has been opened, the vape juice degrades as its chemical compounds interact with light and air. Nicotine begins to oxidize and turn darker due to its exposure to oxygen. 

Vape juice that has been stored for a long time, or is quite old, will have three distinguishable changes:

• COLOUR - As mentioned above, the e-liquid will change to a darker colour, possibly yellow or brown. This depends on the ingredients, nicotine content, and how it has been stored. 
• STRENGTH - Much like PG and VG, nicotine is volatile and degrades over time. This means the potency of the nicotine will decrease. 
• FLAVOUR - E-liquid that has been stored for an extended period will lose its distinctive flavour. You'll still be able to detect a particular sweetness or fruitiness; however, the specific flavour notes will be lost. 

Some vapers like to practice steeping their vape juice. Steeping involves letting e-liquid sit idle in storage for a while to allow the flavour mature. However, vaper be warned, you can also oversteep your e-liquid to the point it becomes either too weak or too strong, resulting in a less than desirable flavour when vaping.