I use fan jets all the time,if you have pencil ones and the windows bead it will take longer,try a normal window at home but don't take too much time on it,like Tosh says wash and rinse within say 20-30 seconds on your standard window unit and see the results later,alot of people take far too long on a window because they worry about the results but if you give it a good scrub and rinse a little it will be fine,don't bother turning the flow up for rinseing and down for scrubbing,just leave it on the same setting,I think you are worrying too much and thats why you are using alot of water,on a normal 3 bedroom semi I would use 10 litres roughly but I have a tap for saving water and use fan jets that give a decent rinse once down the pane.