It takes zero time because no domains need to rearrange when a 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 magnetic are loops than run through the interior of the magnet and then loop back outside. The “poles” are just where the field lines 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.