Hi,
We have a few commercial offices on our books that sound a similar size.
Initially, you need to ask whether they have an idea of how many visits they need. Both of our have 2 midweek visits (Tues/Thurs) with a weekend visit as well.
We take a look around to get a feel of how long we think it'll take to do, then quote at your standard rate.
When putting in the quote, don't put # hours on it. As you become more familiar with the office (and a clean office is easier to clean than a dirty one) you will find you spend less time there, but still retain the same charge.
Say you worked out @ £12/hr;
Tues = 1hrs, Thurs = 1 hrs, W/end = 3 hrs, you can quote for £60/wk
After a while, you may find that you actually work 0.5hrs on Tues & Turs and 2 hours on a weekend. That brings your hourly rate to £15.
That's how some (not all, I hasten to add!) of ours have worked out.
And don't forget to agree to be able to add a few extra hours on the initial clean.
Good luck.