I would say no more than every 4 weeks cause remember the Cellophanes have a lot of 'cones. But like so many other products you'll have to determine what's best for your hair.
Perhaps if you did a clarifying treatment in between you could do so more frequently (that's a colorshines newbie guess) I only cellophane every 4 weeks even though it washes out after about 2 1/2 with my daily CO wash.