Question: How fast to rotating kerr black holes spin?

Rotating black hole


A rotating black hole  is a black hole that possesses angular momentum. In particular, it rotates about one of its axes of symmetry.

Types of black holes
There are four known, exact, black hole solutions to the Einstein field equations, which describe gravity in general relativity. Two of those rotate: the Kerr and Kerr–Newman black holes. It is generally believed that every black hole decays rapidly to a stable black hole; and, by the no-hair theorem, that (except for quantum fluctuations) stable black holes can be completely described at any moment in time by these eleven numbers:
mass-energy M,
linear momentum P (three components),
angular momentum J (three components),
position X (three components),
electric charge Q.


These numbers represent the conserved attributes of an object which can be determined from a distance by examining its electromagnetic and gravitational fields. All other variations in the black hole will either escape to infinity or be swallowed up by the black hole. This is because anything happening inside the black hole horizon cannot affect events outside of it.
In terms of these properties, the four types of black holes can be defined as follows:

Formation
Rotating black holes are formed in the gravitational collapse of a massive spinning star or from the collapse or collision of a collection of compact objects, stars, or gas with a total non-zero angular momentum. As all known stars rotate and realistic collisions have non-zero angular momentum, it is expected that all black holes in nature are rotating black holes. Since observed astronomical objects do not possess an appreciable net electric charge, only the Kerr solution has astrophysical relevance.
In late 2006, astronomers reported estimates of the spin rates of black holes in The Astrophysical Journal. A black hole in the Milky Way, GRS 1915+105, may rotate 1,150 times per second,[2] approaching the theoretical upper limit.
Relation with gamma ray bursts
The formation of a rotating black hole by a collapsar is thought to be observed as the emission of gamma ray bursts.
Conversion to a Schwarzschild black hole
A rotating black hole can produce large amounts of energy at the expense of its rotational energy. This happens through the Penrose process in the black hole's ergosphere, an area just outside its event horizon. In that case a rotating black hole gradually reduces to a Schwarzschild black hole, the minimum configuration from which no further energy can be extracted, although the Kerr black hole's rotation velocity will never quite reach zero.
Kerr metric, Kerr–Newman metric

A rotating black hole is a solution of Einstein's field equation. There are two known exact solutions, the Kerr metric and the Kerr–Newman metric, which are believed to be representative of all rotating black hole solutions, in the exterior region.
See also
Black hole spin parameter
Kerr black holes as wormholes
BKL singularity – solution representing interior geometry of black holes formed by gravitational collapse.
Penrose process
Ergosphere
Black hole Spin-flip
Black hole bomb
Stellar black holes


Further reading
C. W. Misner, K. S. Thorne, J. A. Wheeler, J. Wheeler, and K. Thorne, Gravitation (Physics Series), 2nd ed.    W. H. Freeman, September 1973.
Melia, Fulvio, The Galactic Supermassive Black Hole, Princeton U Press, 2007
Macvey, John W., Time Travel, Scarborough House, 1990