Of course every student is very different, but a very rough number to plan on working at home is 10-15 hours per week in your online course. If you are remediating a college preparatory class, you might estimate closer to 15-20 hours per week.
Remember this is for one class, if you are taking two classes you will need to double the amount of time you need to set aside.
Although you can work on your class at any time that is convenient for you at home, remember that you are taking a class in 6 weeks that is the equivalent of an 18 week course during the regular school year. Plan to commit ample time to you studies in order to be successful.