Well the factory says 100,000 miles. But.... if you wait that long, you're very likely to break off a plug. Carbon builds up in the plug threads and locks it in, and the two piece design of the plug provides the place for it to break off when you try to take it out.
The best preventitive maintenance is to remove the plugs (change them if you like) clean the threads, put some never seize on them and reinstall. If you do this every 20 or 30 thousand miles, you'll never have to worry about stuck plugs.