Many of the reports in the press were saying it was a 4-digit pin. But it's feasible even to brute force a six-digit PIN in the way I described, it would just take several weeks.

Is there any way to know the length of they pin before bruteforcing?

I use a larger than 8 digit pass code on my iphone. It looks different.

Using a default 4 digit code, you end up with a numeric pad and four boxes. If you use a longer than 4 digit code but stick with numbers, it gives a single box to enter the pass code in but presents a numeric pad. If you use letters at all, it switches to a qwerty-ish keyboard and a single box to enter the pass code.

I'm not sure because I'm not running iOS 9, but from screenshots on the web it looks like the length of the passcode is displayed on the lock screen.