It depends on what is going on in my life & head at the time. Sometimes I can only stick with something for 15 minutes. Other times i can spend hours.
Knowing that I have another day set aside makes it easier to stop the project when i have other demands.

Think of it like a school schedule. First period Mon/Wed/Fri is Art
Tue/ Thu it is cooking. At the end of the period you know where you will go next and know when you will be in class the next time. (It might be a different time but will still be the same class on the next scheduled day)