hi simon
definitely day rate
depending on what you`re day rate is of course, the fact that you`ve asked this question leads me to think you`re relativly new,.... apolagies if wrong.
when i look at a large domestic job i split it up into houses, if i think it equates to 9 houses i price accordingly, other things to consider is if its a first clean it could intail scraping, taking even longer than initially thought so take that into consideration. another thing to consider is weather its a one off clean or a regular clean. if it`s a one off go in higher than you would if you were to do regular
thats the way i do it, others may offer different advice, good luck hope you get it
tony