Following up on the article in which I addressed the fact that sometimes it is not wise to clean a gun, this article addresses my recommendations as to how frequently a gun should be cleaned.

The Short Answer
A good rule of thumb is to clean a gun between each firing session.  Generally speaking, I can’t really think of a good reason not clean guns that frequently, however the information below is included for the sake of completeness.  At best, a dirty gun won’t function reliably and will wear out prematurely.  At worst, it may explode if the barrel becomes so clogged that the bullet can’t exit.

The Long Answer
1. Purpose of the gun:
First, there are special considerations when it comes to cleaning self defense guns.  This is because a gun that has been disassembled for cleaning may have been accidentally reassembled improperly in a way that would prevent it from functioning.  A failure to fire or failure to feed might be a minor annoyance when target shooting, but could prove fatal when using the gun for self defense against a home invading criminal.
2. Type of gun:
Next, the type of gun needs affects how often the gun needs cleaning.  Gas operated shotguns, such as the Remington 11-87 and Mossberg 930, have a gas piston system that needs to be relatively clean in order to function reliably.  The same is true for gas operated rifles, such as the AR-15, and gas operated pistols such as the Desert Eagle.  On the other hand, pump action firearms such as the Remington 870, Remington 7600, or Mossberg 500, will tend to function even when quite dirty (although it is obviously better to keep them clean).
3. Type of ammunition used:
The type of ammunition used will all also determine how often a gun needs to be cleaned.  For example, Wolf brand ammunition has a reputation of being “dirty”, and guns that fire it may need to be cleaned more frequently.  Also, non-jacketed ammunition will tend to cause more lead build-up in the barrel, compared to ammo that is copper jacketed.  Specific to shotguns, I’ve found that sabot slugs (which have a plastic “shoe” surrounding the projectile) leave the barrel much cleaner than ordinary rifled slugs or shot.