Depends on the e-liquid and the manufacturer. I have some that will stay the same if I leave them out to sit in a cool dry place for a for a week or two and then I have some flavors that will totally change in a matter of days. I use a quick steep method where, soon as I receive a liquid I take the top off and let it sit for about 3 hours with the top off and have the bottles sitting in hot tap water for about 15 minutes as hot as I can get the water till it cools off to about room temperature and place the bottle back in hot water again for about 3 times over and that's my method. I make sure the water is all about 75% the way up the bottle but not the flood it.. After this method I will see that the liquid has darkened up a little and sometimes even thickened up. But again, it all depends on the e-liquid.