I've said it elsewhere, but will repeat it just for you
I always weigh up initially (in my head) how long I reckon it's gonna take. Then, multiply that (number of man hours) by my hourly rate. You must remember, though, to allow the right amout of time for kitchen areas, stairs, toilets, etc . .)
Golden rule is to not specify the amount of time on your quote. I no longer do this for any of my jobs (commercial or domestic). Quote the price, list the tasks, areas, etc, etc . .
If you present just the cost, with enough wording to justify it, they then have only one thing to make a decision on. If you start breaking it down to 1/2hr for this, 1 hr for that . . . . you're just giving them something to argue with.
The good thing is that, if you initially work it out (for example) at:
£10 ph * 4hrs * 5days = £200wk
you could soon find it only actually taking you 3 hours . .. so you end up with:
£200wk / 5days / 3hrs = 13.33ph
That's how I'd do it, but I seem to have a 'feel' for what is acceptable for each builiding, so that I and the customer are happy with the deal.
Good luck.