or you could charge by the metre.
Have 2 people with a rotary machine each carving through 200m2/hr at £1/m2 and make £400/hr
if its got furniture cover you will only clean about 40% of the total area.
I personally wouldn't charge by time unless it is a huge maintenance contract of 20,000m2 or similar on a monthly or quarterly basis.
Remember to factor in time for site meetings and inductions, H&S Accreditation, Risk Assessments, Method Statements, Staff Training etc as these can all be asked for. Sometimes after you have done the work and even after years of service so make sure you have a little extra in the pot so you can supply this.
Also be sure to find out their payment terms are as some can be 90 days. Always get a Purchase Order for the work. NEVER do work without it. Also make sure that you satisfy all their vendor requirements before starting any work as it is often used as a stalling tactic for settlement.
Ensure you have a contract for each job ensuring you still get paid should they late cancel or you cannot gain access because the security guy wasn't informed at shift change or didn't bother to read his email. Also have paperwork in place for a person refusing entry to sign off on the fact he isn't granting you access.
