I have a Real Estate agent that I work with who is in my networking group. I've had a decent amount of experience cleaning move outs and getting them ready for rent or sale.
I can tell you that the work is very detailed, even on the "cleaner" smaller homes. Especially if they have sat empty a while as the spiders, bugs, mice and other vermin take over BIG time.
Most average sized places will take at least all day if not more depending on the job and how good you are with detail. It is nothing like cleaning regular residential. You have to think of everything - like cleaning the inside of the oven, windows, wipe down cabinets inside and out, window tracks, baseboards, closets, shelves, and likely sprucing up the outside with sweeping, if they need it.
Totally different ball game , but worth it if you charge right. I'd say go hourly if at all possible. A lot of times there is much more than first meets the eye with these types of cleaning jobs. The one's that have been sitting empty take the longest...