One of our customers had someone still his bank account information and crated checks that we cashed at our bank. The signature was identical and we had no way of knowing they were forged items. What is the banks liability here and if we do have to reimburse the customer how long do we have to do that?

The bank has full responsibility for cashing forged checks.

Any limitation on how far back you have to go will be found in your state's Uniform Commercial Code and your deposit agreement. If youy aren'y sure you should ask your legal counsel to review.