Jared,
Decay resistant species such as black walnut "can" be utilized for lumber assuming the tree is recently dead (dead standing) or recently uprooted from the stump due to a weather-related event. Recently dead means the wood is still solid and intact, and unaffected due to decaying processes.
But how long can the tree be dead and still be useable for lumber? Well, the answer to that question is really simple: if the wood is no longer solid, but rather it is soft and doty, you've obviously waited too long.