I once found a $200+ pair of sunglasses (Oakley Juliets) under a tree in the Yosemite backcountry. They were moss-encrusted and half-buried in the humus but still perfectly serviceable once I gave them a quick wash/polish.
Actually, I've been able to hang onto my most recent pair for well over a year now. The trick is never taking them out of the car. The way they get lost is you wear them from your car into some house and take them off.