Also depends on what you are doing and what marker you're using. For woodland players tend to prefer a slightly tougher shell so the balls can go through very light foliage, and some markers can be harder on the balls so you're more likely to get breaks.
A quick test is the drop test - stand on a hard surface (e.g. concrete), hold the ball out at arms length then just drop it. The number that break on first impact gives you an indication of general toughness of the shell. For rental use, I was told 4-5 breaks in 10 drops is about right. For tournament use, you want at least 8 to break out of every 10 drops.
I'm happy to be corrected by anyone who knows better as to the recognised "correct" percentage of breaks, it's been an AWFUL long time since I had to check paint quality!