I'm not sure it matters much how long you boil it, but it might be a good idea to boil it a few times, using fresh water each time. You can also put a bunch of carbon in your filter when you add the wood to your tank to help remove any tannins that are left to leach out.
Sometimes boiling alone will not do it. Soak for a while afterwards. I've been soaking mine for about 3 and a half months. It still leaches tannins out, but a lot less than it did originally.
Submerge in boiling water, let sit and change water daily until water comes clear. Like making tea with the same tea bag over and over again unit no more tea.