Hi Mark
I have been cleaning windows using ropes for six years, we are known over here as abseilers or RAT's (rope access technicians). Estimating is done by working out how many windows you can comfortably get whilst on a drop, then working out how many drops there are. Estimate how long each drop will take then times that by the total amount of drops, you then know how long it will take so you can apply an hourly rate. We always work in pairs, there are many ways of anchoring your ropes, eyebolts, structrual steel, portable anchors, cradle rails. If you are looking to carry out rope access then get the proper training, we are all IRATA trained.
Hope this helps
Jonathan