It sort of depends on what you mean by shelf life. An electronic cigarette sitting at a gas station is not the same as a bottle of nicotine in a pg or vg solution. With respect to just bottled nicotine, storage life is about 1 to 2 years (PG lasts longer than VG).
1 to 2 years at room temperature.
Kurt the chemist and a few others have reported over 5 years with no degradation or flavor change from their freezer stock.
I just finished my 10/2013 RTD (from the freezer)and it was as good as the day I received it.
Oxygen is our friend but no friend to our nic.
I asked Essential Depot's owner how long their PG & VG are rated for and he said at least 5 years if it's kept cool.
Leave a bottle of nic in the sun or on a table or shelf in a hot room and it'll probably be nasty in a few months.