I don't know. And my sense from our occasional conversations is that he's not entirely sure himself. But, at age 62 (he turns 63 in July), he has loads of energy and plenty of interests. I expect him to take on multiple gigs, in a variety of areas, from media commentary to education.
If I were a betting man, I would bet heavily that one of those gigs looks very much like the sort of deal former British Prime Minister Tony Blair recently made to advise a California venture capital firm that invests in environment-friendly technologies. Schwarzenegger has invested much of his time in office promoting just those sorts of technologies--and has made himself an international expert on the subject in the process. He could make some money while pursuing work he finds interesting -- which is what just about everyone looking for a new job wants.
Also, Blair and Schwarzenegger know each other pretty well -- they even signed an agreement on climate change when both held office (as shown in the above video).