Usually once or maybe twice a year. Any deep inspection is usually done in the spring - for splits & swarm management & just plain checking. Otherwise, as MB says - only if something appears to be wrong. I sit & watch them frequently - at least a few times a week, which is how I determine which ones may need some extra attention.
In my experience, the worst thing you can do to a great hive is tear it apart for no good reason. Or just about any hive, for that matter. If you must, try to leave the brood nest in the same configuration you found it. That way the bees won't have to waste as much time re-configuring it.
Using a spare empty box is an easy way to accomplish this. Start with an outside frame, and place it in the empty - repeat. This is also a good way to cycle boxes for maintenance. Of course, the more spare empties, the better - a full hive's worth comes in very handy.
When extracting, I usually put the empty supers away in a nine frame configuration, with new frames in slots 3, 5 & 7. I may set aside one full super of comb per hive, to go on first in the spring, to "get them rolling". Some years they get almost full bare frames supers - just a frame or two of comb in the middle. That just depends on how much comb I have, and how many producers I run.