I wondered this myself, since the singularity is assumed to be a point, thus it should spin as fast as possible.

But this appears to be more complicated:

https://physics.stackexchange.com/questions/183832/typical-r...

https://physics.stackexchange.com/questions/174247/is-there-...

Taken from one of the stackexchange posts, this is quite interesting: http://www.nature.com/news/spin-rate-of-black-holes-pinned-d...