It should be at least one week plus the amount of notice required to terminate as in a week for the parents to think about how the rate increase or whatever changes will effect them and then time for them to give notice if they intend to terminate instead of accept the changes. 1-2 months is the nicest thing to do so that parents have time to start budgeting for the change. Many families live paycheck to paycheck and an increase of $10 on the bill can make a difference.