For me the Reflectex lasts about two years in southern CA when used outside the glass. Mostly use it in storage but have used it on RV when it's very hot and I'm camping. The strips of Velcro last 6 months to a year. I always have supply on hand to refresh the bad strips. Upkeep on it is no big deal. This keeps the window glass much cooler than without, and much cooler than the reflectex inside.
My suction cups exposed to the weather is what did them in. Once wind knocked off one of the panels, they'd fall to the ground and get damaged. One was always pulling away from the glass.
All in all in hot climate firmly believe reflectex should be on the outside. For cold, inside may be better (and simpler).