Much better to cure the cause of the damp patch. De-humidifiers aren't cheap to run if there is a continual source of humidity.
I have used them, my advice is that if you aren't going to plumb in a waste pipe, get one with a bigger tank, otherwise the things are auto-off with a full tank too often.
The first 10 days or so, expect it to run nearly continually as the carpets, walls, furniture etc release their water content.
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