I've been wondering about this for a while, and from what I've seen, it's anywhere from 20 years - 50000 years.  I think 200 years seems good, but the state of the relaxation chamber seems a bit good for 200 years.  The same thing goes for paper and wood.  And what about Chell's clothing?  I'd like to hear what you think, and why, if you are a Portal fan.