It depends on what you are going to do. Is it just follow/unfollow/like/comment? Or posting, coming up with headlines, etc.?
If doing just f/u/c, mostly automated
Charge 5 hours per week, minimum, at your local rate.
If doing posting, headlines, etc., mostly automated
Charge 7 hours per week, minimum, at your local rate.
If doing everything manually, then charge for the hours worked.
Remember, if you have the experience, it will be cheaper to hire you, then have someone full time at the office doing the same.
Now, if you don't have the experience, then, charge accordingly.