It's really a question of how long it will take to turn over the property between bookings. Make sure you have long enough (or enough people to help).
We do everything ourselves, for a two bedroom house. I reckon it takes me about 8 hours to clean, wash, iron and put the place right after each guest. This varies depending on things like the number of guests, small children, dogs, etc.
Getting flowers (from our allotment), putting together a welcome basket, cleaning the shower screen - all take time. Some guests will use every towel in the house, even those hidden in the closet!
I would suggest asking your friend how long they take for each booking, then add a couple of hours as you are new to the job. Then work out an hourly rate you'll be happy with and see what they say.