Not as tough as Chuck Norris or Mike Ditka.
speaking of which I lost my S**t when in the expendebles II chuck norris rocks up and tells a chuck norris joke
I've found that my Canon front elements are very tough, far more than the expensive protection filters that go in fornt of them. Over 7 years of professional wedding phootography, I have only scratched one lens element. My 16-35IIL and that only showed in photos when I pointed it at the sun.
But and here's the big BUT....when a lens is damaged, the front element is really expensive to replace. My front element on my 16-35IIL cost me £325 to get fixed.