We'd love to give a definite answer on this but it depends on so many factors!
It will biodegrade in the natural environment when exposed to moisture, oxygen and micro-organisms, the levels of which change all the time. When tested to ISO14851; exposed to an environment similar to waterways in the countryside such as streams, rivers and lakes, the vast majority of the biodegradable content biodegraded in only 5 weeks. Warm and wet conditions are the best, so it going into our sewage systems is the best place for it.
A good analogy is the fact that a leaf will take much longer to biodegrade in an arid environment than in a damp swamp.