As long as stored in dry place, how long can I expect an unbuilt kit to resist weathering and possible warpage...environmental damage...that kind of stuff???

Long story short... long enough you could forget you had it and then once you found it again, build and fly it.

I have a Sterling Corsair kit that is 10 years old and still looks new. I've also seen one year old kits that are trashed from care-
less storage: heat, humidity, mice, etc. The first thing that does
start showing it's age first, though, are any decals that might
come with the kit. If you are planning long-term storage, remove
the kit's decals and preserve them between acid-free paper rolled
up in a PVC tube sealed at both ends. I have water transfer decals from the 70's that were kept this way and they still work!
Here's a shot of the 10 year old corsair kit, that I plan to finally build this winter!