I don't think it should take more than a few days if it is exposed directly to the dehumidified air (i.e., it's out on a stand in a room with a dehumidifier running and not closed up in its case).
I recall a study that was done for 20th Century Guitar magazine many years ago in which they subjected a solid wood acoustic to high, then low humidity. They used a wood moisture content meter to gauge the extent and speed with which the wood was gaining, then losing, moisture. I recall that it took not much more than a day for a guitar to substantially dry out after being exposed to high humidity.
David
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