I have about 50 TFA flavors I'v never used. I bought them about a year ago. I also have a leage botle of nic/pg, pg and vg but I assumed those arn't likely to degrade.
Rubber dropper caps are hell on flavor volatiles. They evaporate right out the dropper cap. Use tight non dropper flat caps for longevity.
I toss my flavor bottles in vacuum jars and suck the air out with my sous vide vac sealer. Less air means less escape routes for aroma molecules over time.
Jesus Christ on a stick! I just put Dropper caps on all my 4oz flavorings last month...
About a 2 year shelf life for the pg, vg and nic, as long as they have been kept in a dark and cool location. You can also store then in the freezer for about 3 to 5 years, if not longer.
As to the flavors, if in glass, I would feel ok with going 2 years also, if stored properly (same as tbe vg, pg and nic). If in plastic, I would toss them. Seems many flavors are not freezer friendly, so freezing them doesn't seem to be an option. Only buy what you know you can use in a year.