When it comes to the question of how often windows should be cleaned, the answer is not a straightforward one, and one that will depend on many factors. One of the main considerations is personal preference, which may also be tied to the issue of budget. If you want your windows crystal clear then you obviously need to have them cleaned on a very regular basis, whilst others may be perfectly happy as long as you they can see out and will happily wait until windows desperately need cleaning before taking action.
There are other factors to consider too. Buildings located in an area that experiences a great deal of heavy rain or snow, or those which are exposed to a large volumes of passing vehicles, are likely to require more regular cleaning than those in dry or less busy areas.
The main reason some businesses might choose to make more regular use of cleaning contractors is that a sparkling facade makes a good first impression. Shop windows need to look clean, both to show off products and to look inviting to customers, and many financial business like to have very clean windows for the transparency and attention to detail that will be inferred.
In general, the entirety of a commercial building needs to be cleaned two or three a year, but for frontages on retail stores, or offices lobbies, you may want to consider employing the services of one of the many commercial cleaning companies every four to eight weeks. Ultimately though, the decision is up to you, and depends on the importance of clean windows for the projected image of your business.