Depends. Short answer is 1-2 years.
Long answer depends on what it died from, how long it took to die, and how much fungus has moved in. It also depends on the market which the log will be sold for. If it was a pallet log and and isn't too rotten than it won't lose much. If it was a Grade 1 and stood dead for a few years it won't be suitable for that market anymore. Stains caused by fungus will keep it out of any high value use. If decay is advanced then it isn't worth much except for fire wood.