I don't know how much this will help, but I have a friend that did this for one school year. It was all over the board. Sometimes she would know she was working certain days to cover days off, etc. Sometimes she was called the night before, sometimes morning of depending on when someone called out sick. It was really hard to predict. It is similar to a substitute teacher. She did have the option to decline, but that was frowned upon. It was too unpredictable for her so she only did it the one year. I would ask the person hiring you to outline more information about the requirements of the position and ask how many hours this position usually averages based on the previous two or three school years, etc.
Again, not sure how helpful that was, but good luck to you :-)