I know, I know, we are just the lowly substitute teachers. We do not have to write report cards, have student conferences, get to school early, or stay too late. We actually don't have that bad of a deal here. Having said all this, I think we are entitled to one prep a day (if it is in your schedule). Anything in access of this one prep is just gravy and you do not need that much time to snoop through the teachers drawers.
I've had insane amounts of preps before. Often times the teacher will know they have a lot of preps and leave you work to do. I've spent a lot of time with the laminator and even more time snipping out laminated items. But sometimes, the teacher calls you in last minute. She barely had time to throw together a haphazard lesson let alone plan for what you would do for her on your preps. In these cases, and I know it will be hard, let the office know about your insane amount of preps.
The office may give another teacher who has been putting in a lot of extra time in extracurricular activities some time off from the classroom and get you to cover for her, they may get you to do some office work, or they may get you to just help out in another class. Whatever they get you to do, you do it, with a smile on your face. Because, you love teaching, even if that means cleaning the puke off the floor.
And, sometimes they will find nothing for you to do and you get to sit back with your feet up eating all those bon bons with only the guilt of you cheating on your diet on your mind.