When enough dust particles collect on windowpanes light is blocked. Removing those particles will brighten your home and your day but how often must this be done? Interior window cleaning is generally easier because you don't have to stand on ladders, avoid shrubbery or trees, or plan around bad weather. It's the exterior panes that many times need more attention.
Depending on your region the annual number of times you should clean the exterior panes of your windows can be arbitrary. However, to halt excess dust building up, scrubbing them seasonally at least three times a year would be a good choice. For example:
That last point is important. After all, cleaning the exterior of windows and then pruning bushes and doing other exterior work may push debris back on the glass ruining your effort.
Meanwhile, once you've decided on a schedule realize you don't have to be a professional to do a good job. All you need are the right tools so here are a few suggestions:
Finally, clean your windows and let the sunlight in but don't let it hinder the process. Cleaning windows on a hot day or in direct sunlight will cause them to dry before you even squeegee it. If so, you will be forced to start over wasting water and your time.