Wilderness Systems advises owners that the boats have an expected lifetime of 8000 hours of exposure to sunlight. Sounds like a lot, but it adds up fast if you boat is left fully exposeed day after day.
If you are "forced" to store it outside, I'd have it under a tarp to block sunlight on it AND store it so that the boat doesn't warp over time.
BTW, after the 8000 hours, the plastic becomes brittle and is not repairable.