Hi,
From my limited knowledge...
It really depends on how long your proj is going to be on hold.
The advantage of keeping it under water is that tannic acid can continue to leech out, so when you finally use it in you tank, it would not discolor your aquarium.
Since your piece is a mangrove root rather than a tree stump, it should be hardier and less prone to softening, I would suggest you continue to soak it till most of the tannis are released, then let it dry and store it.
Btw, here's a link that describes the difference between drift wood and bog wood. I've known some hobbists who are confused between two.
http://www.drhelm.com/aquarium/decorations.html
Cheers
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