A geldings sheath should be cleaned at least once a year to every six months. Every two weeks is a big NO! This can cause extreme irritation to the sheath.
There are several different theories posted but they all are a little mixed up (most are valid).
Once you have cleaned your horses sheath it needs to be done annually. Once you start you must continue because you have caused a disruption in the sheaths natural cleaning abilities. Studies do suggest that, if you cannot do it annually, don't do it at all or get a vet to come out (it is really not that expensive).
You may think your gelding is fine without having it done but if you were to reach up inside his sheath, far enough, you will find little 'beans' that can be excrutiatingly painful. Failing to remove the beans can cause several different kinds of cancer and you may regret not doing it, later.
Yes, males are equipped with a natural cleaning ability in their sheath. But it was not designed to withstand being kept in a stall 12 hours a day or even 24 hours a day. It is not equipped to tolerate shavings finding their way up inside the sheath and it was not meant to withstand hours of training and sweat everyday!
There is a sheath cleaning product available that works great but is very expensive. The best thing to use, Ivory soap! It is cheap and recommended by any vet as it is very mild and will cause less irritation. Please remember, whatever you use, to thoroughly rinse to make sure all soap residue is gone!