To be honest, by the sound of it you will price over the odds for the building.
18 floors is quite high, but how wide is the building.
We used to do something similar in Bradford where the cradle needed de rigging and re rigging, one part of the building took two days and when we started we used to book the cradle company to change over on day three - giving us a bit of leeway for bad weather etc, and we always had the insides to clean whilst we were waiting.
Depending on what type of cradle you are using will depend the length of time it takes to clean the windows. Some are electrically operated (skyclimber etc) and some are manual(Tirfor etc) - and 18 floors would be hard work with a manually operated one
If you need any further help with quoting - e-mail me
darran@purple-rhino.co.