Not much, since I don't store my juices in the fridge or anywhere special, other than somewhat out of the light. Mine are in my office, which gets pretty hot this time of the year if I am not at home. High-nic unflavored liquid, OTOH, I do store in the amber glass in the freezer, although that also might be overkill. A few days out of the freezer or fridge is not going to do much one way or the other.
Even in the fridge, nic will react with O2 in the PG or VG, but there is not much present in it. Flavors can go bad
through water reaction (hyrdolysis), especially some fruit flavors, and they will go whether they are in the fridge or not. Most flavors will just fade, and so fridge MIGHT slow that, but it might not too. People say keep them in the dark. The reaction with light is mostly due to UV light, and there is not much of that inside a house, unless you tend to use a lot of black lights. Visible light is not the wavelengths needed for reaction. In the sun is a different issue, but then most plastics and glass block UV...but glass will turn UV into heat, so there is the heat thing, and too much heat is not great for flavors. But 90 deg is not going to do a lot in a few days.
Realize that most vendors just store their juices on shelves, not in a fridge.
