The rubber will degrade but not in the biological sense. Contact with
microbes, bugs and other stuff in soil will have no effect but
eventually sunlight and oxidation in air will turn hydrocarbon rubber
to dust. It would be hard to put a number on but I doubt if it would
degrade faster than wood chips. I've had cheap bicycle tires on bike
in garage fall apart in a year but I've seen thrown away tires in a
field last for years. Guess it depends on how good the rubber
antioxidant package was. I've been assuming these rubber chips are
ground up tires. Some of the zinc/sulfur curing agents were given a
bad press by environmentalists. Unfortunately you cannot recycle
rubber back into tires.

Frank