My normal regiment is once a week (Same for the cars). The extent of cleaning is often dependent on the amount of riding and the conditions of the previous week. Often times it can be a simple a running a California Duster of the shiny parts.
To me, the reason for cleaning is three fold:
1: I like nice stuff. I paid, what I consider to be, a large sum of money for this vehicle and would like to keep in in as nice condition as I can.
2: Longevity. Cleaning any vehicle forces you too look at places that would often go unnoticed. You typically catch worn out components sooner. You also prevent wear by removing contaminants from surfaces, gasketed areas, drive components, etc..
3: Re-Sale. I have short term buyer syndrome. I have yet to have a car, bike, or ATV that I have kept over 5-years. The physical appearance of the vehicle at a sale holds almost as much weight as the maintenance/'service record...Sad but True.