I would expect something that was biodegradable to biodegrade in less than 700 years. If you have that sort of time, plastics are sort of biodegradable as well.
Being biodegradable doesn't mean that it will degrade in any circumstances. Lumber in a house is usually protected from many biological degradation agents, even if it is not chemically treated. If you took that same wood and threw it in a pile exposed to rain and bugs, it would be gone in less than 100 years.
Are you listening to yourself? Are you really trying to say that wood is not biodegradable? Lolwut