Pretty quickly now so long as it's not yet in leaf/flower. Or anytime while it's leafless and dormant in winter. The reasoning is that when in leaf, the plant needs to take up lots of water to make up for what's being transpired (evaporated if you like) though the leaves - the disturbed and broken roots probably can't keep up with the demand, so it's likely to wilt and then die (depending on the level of water shortage). Doing it while dormant means that leaves/roots stay in step with each other while the roots heal and grow new.
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