Never had a D-loop break.
I bought a bunch of BCY 2mm toop material for a gal on archery talk (RedHDCharm). Very reasonable, and high quality stuff.
I tie a lot of loops on for myself and my buddies, and we've never had one fail. We replace 'em once or twice a year, and wax our loops and strings every ~75-100 shots.
The key is melting the correct "ball" on the end of the loop material, then waxing before tightening the knot. Then just examine it each time you wax, and replace when it looks warn.
I'd suggest going over your release for sharp edges or burrs if wear or breaking becomes common.