What i have done is give the frames a quick soaking starting at the top obviously, then once the bottom ones have had a soak i've started back at the top and found it has only taken me about and extra 10 mins to do.
It obviously depends how much muck is on the frames, but you could also work something out on your hourly rate.
If you work on a rate of £20 an hour and you think it will take that long then charge them that.
If it takes less than that then give them a fiver back and say it didn't take aslong as you thought.
I've done that before a few times