I looked for an answer on fansites and I did not find. So I went to a spider cave and tried 100 times to broke a spider web, 42 times I broke them. So we have here a percentage of 42%, but it's not exactly, to have better results we can test in many caves and 1000+ times. I tested in GS cave in Forbidden lands.
Attention, this is not a official answer, I got it with 100 attempts. We need much more attempts to got a most accurate answer.
If you use a firebug you have 100%. It always put fire in spider web.