I've only left the paintings up for a couple of months. I have to agree with Leslie, one of the problems is keeping little hands away from it and scratching it off.
I just put a "squirt" of dishwashing detergent into the tempera and that seems to work well. When we clean it off, we just use water and borrow the custodian's squeegee and dry with paper towels. It can also be scraped off, but that it a bit more tedious. Hope that helps.
Anne C-H in Illinois
Does the dishwashing/tempra paint
last the whole year without chipping or fading?
I found that this lasted except where the children scratch it off. I have a fifth grade group of students, who are in charge of fixing the scratches.