I would provide at least 30 days advance notice. This will serve as the "warning" to those individuals that rely on the images for their recordkeeping (as opposed to maintaining a check register as they should). State statutes I have seen do not require check images be provided, only a periodic list of the items that cleared the account by date, check number and amount. You will want to check your state's statutes before making a final decision.
Life without Jesus is like an unsharpened pencil - it has no point.