Depending on the size and color of the wood....probably months/years.
Boiling the wood might speed things up, but I've never tried it. Maybe someone else here has.......
On the way back home down the coast, many of the local people sell driftwood from the brackish swamps. I'm not sure what exact type of wood it is, but it is certainly weathered.
Would that be a good source of driftwood for the tank if soaked long enough to get rid of the salts?
To answer some of the questions here. Drift wood never stops leaking tannic. I have some pieces that are over 20years old and they still turn water brownish.
Drift wood good for plecos because they eat it. They chew on drift wood.