Hi Craig,
This is not a myth it is true. WFP does keep the windows cleaner for longer.
One of the reasons behind this, is traditional methods use chemical substances (soap/ washing up liquid/ etc) although windows are squeegee, a sticky residue is left on the window (No you cannot feel this by touching the window). This helps dust and dirt to adhere to the windows.
Because WFP is pure water and no chemicals are added, no residue is left on the window and the dust and dirt cannot adhere to the window as quick.
90% of my clients that start on a monthly cycle switch to a 6 weekly cycle after 3 months. I do charge extra for this and simply explain, there is more dirt on their windows on a 6 weekly cycle than there would be on a 4 weekly cycle, hence more work involved in cleaning them. I have not lost any customers for charging slightly more on these cycles.
I am also glad, you are starting to do well, keep up the good work.
Andrew