No set length of time to boil it.
But you need to know that no matter how much you boil it, the driftwood WILL release tannins into the water. There is no such thing as driftwood that doesn't. However, the larger amount is released during the first few weeks.
So you can boil the wood for maybe 3-4 hours, and then get one of those big 5 gallon buckets and just soak the wood in that. Check the water everyday and when the water looks discolored, dump it out and replace it with new water. Once you get to the point where the wood can sit in there for a week or more without significant discoloration to the water, it is ready to put in your tank.