Why wouldn't it last functionally forever? It's now in vacuum, and eventually will get to whatever temperature the sun can bake it to at that distance from the sun... I'm guessing it'll be there as long as the car is.
Radiation might be a concern. It could possibly embrittle the paper over time.
I'd guess that the temperature fluctuations and radiation would be pretty hard on paper. Zero humidity would probably make it brittle as well.