What is the average length of time to clean 100,000 square feet? I would have to pay employees (how many?) plus the rental of equipment.
How long it will take you to do this job depends on how you are doing it and how soiled it is, which dictates how you should clean it.
If you can bonnet clean (one of the fastest cleaning methods) you can cover about 1000 square feet per hour. The job will then take about 100 man-hours. Divide that by the time period during which you need to finish the work and you can determine the number of employees you need.