It takes zero time because no domains need to rearrange when a permanent magnet breaks in two. The spins in each half are still aligned and still produce a magnetic field.

The idea that magnets have “poles” is a misconception. There are no magnetic poles in nature. This is the meaning of one of Maxwell’s equations,

$$\nabla\cdot\mathbf{B}=0.$$

The magnetic field lines of a magnet are loops than run through the interior of the magnet and then loop back around outside. The so-called “poles” are just where the field lines happen to emerge from the interior to the exterior, or return back inside. When you break a magnet, the field lines simply come out and go in in two new places, so that each half has its own loops and its own “poles”.