I think it depends on each bit of wood. I soaked mine for about 5 weeks, changing the hot water every day as the wood was a dark old gnarly bit that leached loads of tannins. Yes i could have just put it in the tank as it had no problems sinking, but there were so much tannins it would have been unsightly and would have required monster water changes all the time.
The density of the wood will also affect its ability to sink. So when it sinks, its all about personal taste as to how quickly you put it into the tank. I couldn't boil mine, the bit was to big for my biggest pot. I did scrub it a bit however, as bloo said, and kept it in a 45 litre bucket from B&Q (cost a fiver, was good for water changes too).
Squid