I would say that you are offering a service to your customers and it is they who are wanting you to clean them every month or every 2 weeks. Remember that the customer is always right, if they wanted to be cleaned twice a week and were willing to pay for that, then it is up to you as the professional to do as the customer requests.
Dont sell yourself short, it may seem that the windows are clean enough, but when the sun shines on them after a month they most likely will need a clean.
Dont feel guilty about this, it's just part of the job.
Tim