Join Date: Jul 2010
Location: Outside Philadelphia, PA
It is not possible to give an exact amount of time it will take for wood to sink. This depends on the type of wood, how thick the pieces are, and the method being used.
Unless your using a really dens wood, such as mopani, which sinks even when new, you usually need to add some sort of ballast to the wood in order to get it to sink.
You can use nylon string, or cable ties to attach rocks to the wood. It's really not too hard, and works well for most "branchy" types of wood.
If you have something really big that needs a lot of weight, you can get a piece of the red slate that is usually sold at your LFS, and drill it with a masonry drill. Use stainless steel screws to attach the rock to the wood. This is more work, but is almost invisible. You can use other soft rocks that can be drilled.
Another method you can use is to use a plate made of plastic sheet, and drill that. Use stainless steel screws to attach the rock to the wood. Then you can pile gravel and rocks on the plastic sheet to hold everything down. This has one disadvantage. If you remove the rocks or other material, the wood will float up.
I realize the op didn't want to attach rocks or other material to the wood, but in many cases, it's really the only good way to go.