Under the law, you have have 30 days to report a forgery after receipt of your statement, during which the Bank is 100% liable. You have 1 year in California to file suit from the date of the forgery, but the Bank may claim that you were contributory negligent in failing to report the forgery.
So from day 1 to 30 days, you are zero at fault and the bank is 100% at fault.
From 31 days to 365 days, both of you (you and the bank) are at fault, and some compromise has to be made or the case taken to trial to let a jury decide who was more negligent, you or the bank.
This is California law.