The first commercial I did was 200sqm; the full HWE process took me 6 hours of hard work.
It was a learning/turning point for me. I now offer rotary cleaning to these types of (commercial) jobs and would expect to complete in less than half that time.
Incidentally, I recently won the contract from that first job. They still want me to HWE it (not rotary), every 20 wedding events. It should now take me about 4 hours to HWE properly... and there's still plenty of room to improve efficiency.
I'd suggest you cost it as a days work... by my standards it would have taken me a very long hard day (inexperience, inefficient, trying my very best etc).
Do a good job, with a smile, and leave them with your business card.