Well... the last patent that makes driverless actually feasible in the real world will last for 20 years, and for most of that time the development will already be done.

Approval? That's another matter.

More than one company is working on driverless cars. Either most of the patents will be easy to work around by some other approach, or several companies will each hold some of the needed patents and all will license from each other - it might stop someone new from getting in but it won't stop everyone already in. I would expect some combination of both.