The basic rules of e-liquid storage are that it should be kept away from light, in a cool location and somewhere that children or pets won’t get access to it. that is why we created the cardboard tubes to give our liquid more protection.
If it’s exposed to light and air, the nicotine will oxidize and your e-liquid will get darker. This doesn’t change its core properties, but it can be easily avoided with proper storage. If you store your e-liquid in the worst way possible (near a window with the cap off) you could lose a notable amount of nicotine, but in most cases the difference is negligible.
The bigger concern for spoilage is the flavors, which will degrade more quickly in the presence of light and heat. None of this is going to do you any harm, but the flavor will be ruined.
Ideally, e-liquid should be stored in an air-tight container, but as soon as you’ve had a bit of your e-liquid this is hard to achieve. Glass is better than plastic for long-term storage for this reason, though, because plastic is slightly permeable by air.