The official shelf-life, or rather "re-test date", is 2 years for pg and one to one and a half for vg, but as danny and others stated, then it can last a lot longer and it's just a general rule to mark products as such.
In my country where I
buy locally made vg and pg, then they told me that when the date is near the end time of the official shelf-life time, then they just retest there vg and pg and if it holds up to spec still, then it gets renewed another 1 and 2 years respectivelly.
This is also why e.g. on nic from alchem that it dosen't state a shelf-life date, but rather a "re-test" date...
Pg and vg shouldn't go much higher than 25C celsius btw, and can be prolonged further, as kodak and danny stated, by freezer-storage or refridgeration...