Hi liz
As a guide, I estimate 1 hour per living room, hallway or bedroom, 3 hours for a kitchen and 2 hours for a bathroom (just 1 for a cloakroom). This is for a thorough top to bottom clean, wash of all woodwork, inside cupboards, fridges, oven, etc and all interior windows.
Hope that helps, but I agree with Rob, don't tell them the hours, just give them a price but don't forget to emphasise what you will do for the price.
If you feel that the price is going to be too high for them then ask them if they would like an 'essentials' clean instead, no windows and no woodwork and knock something off but warn them their landlord will be expecting the 'bells and whistles' service.
Hope that helps
Diane