6 months...maybe 9. I forget. I think it's labelled on the bottle if I remember right (it's been a while). That's from the date of manufacture, not the date Van's ships it.
Older stuff will still work but it takes much longer to set. Life is extended if you store it in the freezer. Anyhow, my reasoning is that Vans takes a week to ship...it'll probably take you a week to get the tanks ready (if not longer), so if you order when you start the tanks, it'll probably show up well before you're ready for it.