It's usually about 12 months but it really depends on how much transaction history you have on the account. For example, if you write a check from an account that you frequently access with your debit card, it may be less than 12 months.
Even though you may no longer be able to view a check image in Online Banking, you can request a copy of a check at any time. Upon request, we'll provide up to 10 check copies free of charge (per account, per calendar year).