Time elapsed since last window clean. The windows will be much dirtier.
How high are the windows? More height requires more skill. Do we need to use a water fed pole system?
Do you have fly-screens, safety screens, grills,
curtains, blinds- anything that may interfere with the cleaning process? Do these objects need to move to access the windows? Will you do this, or do you want them to do this?
Extra work but highly recommended, as dirt from the fly screens often is transferred to the windows.
Inclusion of flyscreens, tracks, sills, frames or
This makes it harder as it shows up every blemish and surface defects.
These are a whole different animal. Exotic building residues require special techniques and chemicals in order to be removed.
Factors related to difficulty of the job.