In my 10g, about a week, from 8.4 to 7.2 where it remains steady. I just bought two largish pieces of wood for my 29g also and a tiddly piece (but a nice shape) for my lonely Betta in his 5g! I like the yellowing effect, especially for fish who like to live at the bottom of water, like corys...makes it a bit dimmer for them. Many plants will also appreciate not being in strongly alkaline water, so wood can help them also and it is something you might want to tie some java ferns onto, using fine fishing line...they will eventually start to grip the wood and grow and spread along it...look at some of the planted tanks on this site to see how cool that can look. Also, my new wood sank immediately even when dry, while other pieces have taken a few hours.
