I have a piece of driftwood in a tank that is several years old. This piece of wood has been boiled and soaked and is still releasing tannins after having been in my tank for a couple of years. Personally, I like the look of the tannins as it creates a more natural color to the water. Very few lakes, rivers, etc., where fish are found are completely clear. As time goes on the wood is also disintegrating and is taking on a different shape.
I do think that wood will always give off tannins to a degree. I think a lot depends on the size of the wood and the amount of water it is in.
Allie