Fake checks can sometimes take weeks to discover, but if in that time before it bounces, you have spent the money, you will owe it to the bank. Not only will you be out of pocket from the bad check, but you will have to pay a fee for the bounced check and potentially overdraft charges if you have insufficient funds to reimburse the bank.
If you don’t pay the bank within a specified time frame, it may close your account. This could affect your chances of opening up new accounts with other banks in the future.