I suppose that depends on the size and density?
Apparently boiling it is supposed to remove the tannin rather quickly. I read up that, if it's small enough for a pot you own, you should boil it for 15 minutes > dump the water > boil again for another 15 > dump the water > boil one last time for 10. Soaking it like you are, though, will diffuse the tannin. Do you replace the water it's soaking in with hot, fresh water at least once a day? That's what I did for my piece over the course of a week after a quick, 10 minute boil. After putting the piece in earlier today, my water remains pretty clear.
Basically, I guess when the water you're soaking it in doesn't discolor, you're set.