Very high. I think its every 50 steps you have a chance of one shaking. That's how it works for me.
If you use a Repel, and you go in the Grass, it makes finding Rustling Grass easier.
If you want to find rustling grass either to train (Audino) or there could be a specific wanted Pokemon just jump on your bike and stay at that patch of grass and it should start shaking.
Hope this helped! :P