Sometimes it takes hours, sometimes months. I like to do a long boil then a tannin leach for 4 days or so. If its still floating I weight it down with pavers by screwing them to the bottom side of the wood.
As long as it takes to become waterlogged. The harder the wood is, the longer it will probably take to water-log. I have 2 large Rubbermaid trashcans in the garage that I soak wood in for months, or even years until I need it. That way you always have some ready to go. If you keep the can's lids in place, the water won't evaporate.