The account's cash value is updated as soon as IBKR receives and processes the official closing Index settlement value. In the Classic TWS Trade Log you will see the cash settled option closing transactions as an "AUTOEXERCISE" in the Action column with the time of the update from the OCC. If you click on the plus sign ("+") and expand the line you will see the dollar amount of the Credit/Debit in the Quantity column from the expired option.
The cash settled index option's expiration transaction can also be viewed on your Trade Confirmation report in the Trades section with Codes "Ex" for Exercised or "Ep" for Expired. The Trade Confirmation report is processed overnight and will be available after midnight EST. The weekly options that are PM settled Friday are processed after the daily Activity Statement is already run, so the expired option's closing transaction will be in the Activity Statement for Monday (which is available on Tuesday). The Activity Statement Trades section will have the option's settlement price under Close Price (CPrice). On that same Activity Statement there will be a section titled Option Cash Settlements which will also detail any cash adjustments to the account from the settlement of expired cash settled options.