A lotta water changes. I put 4 or 5 pieces of mopani in a tank once and it took nearly a month of huge bi-weekly water changes to get the tannins out. It looks sort of cool though and doesn't hurt the fish. So it's not a big deal unless you just hate the darkness of it.
depending on wood type it will take a very long time for all tannins to leach out and removed with water changes alone. If you have a sump filter use polyester batting semi-tightly packed into the filter chamber. I use this as well as white felt fabric on top of the drip tray and a 5 micron sock tee'd of my pump return. I have the tank filled with wood and have never noticed any tannins. For the first few weeks the batting and felt and micron sock would clogg daily with brown gunk. And after that they still turn brown in a matter of days, but continue to filter for weeks. Water changes will work but you have to suffer thru a ugly tank for months or longer. If you filter it out you just have to go thru more filter washings.