How long does it take wood to fully decompose?

Wood will degrade in months to millenia, depending on the humidity history, access to oxygen, and insolation the particular piece of wood undergoes. Wood fibers that arrive in a waste treatment plant (like toilet paper) are turned into (essentially) soil in less than a week. Termites make short work of most wood, either directly processing the cellulose, or leaving behind the bacteria that will complete the job.