Most windows do stay cleaner longer but the real bonus is that there easier to clean each time due to the frames being cleaner than when using traditional methods,if your needing to clean every 4 weeks you need more work.You don`t need to clean your houses every 4 weeks in order for the windows to stay cleaner longer,the WCS that have a 4 weekly round must just be happy with average paying domestic customers,i clean large domestic and couldn`t charge what i do every 4 weeks,my advice would be to extend the cleaning times but increase the cost per clean but this would mean that you will need to get more work to cover the gaps now left in your workload ie 2 weeks worth.