In the case of U.S. exchange listed options cleared by OCC, IBKR's deadline for providing notification of intent to exercise is 17:25 ET if the option is being exercised prior to expiration. At expiration, notice of intent to exercise is generally not required as OCC will automatically exercise options that are in-the-money by $0.01 or more.
If a client is required to provide notice of intent to exercise at expiration (i.e., OCC removes contract from Exercise by Exception processing due to a stock halt or other uncertainty as to underlying value) or does not want an option that is projected to be in-the-money by $0.01 or more exercised, then instructions must be provided to IBKR by 17:25 ET.