I usually wash large pieces with hot shower for a few minutes and fill the bath and soak them in warm water (unless you have a driftwood bigger then a bath, I have had a few 4ft ones max) for about an hour.
If you really want to soak it further then you could get a tall waterbutt and soak it in there but hot water and warm water will usually get rid of most of the naturally leaching stuff from it (considering the wood is actually aquarium safe).
Nim