I bought a package a few years ago to fashion a clear pick guard for an acoustic guitar - and had a bunch left over. Today I had the brilliant idea of putting some on my chain stay as a protector.
I think it's plenty tough for the occasional chain falling on the stay. I'd say, roughly, it's about as tough as duct tape. Certainly not bullet proof, but virtually invisible.
For the $10 you spend, you'll have enough material for about 3 bikes.