Does anyone know if the result of API request getTotalCumulatedRewardsForUser changes to 0 immediately when that address send a withdraw transaction? Or will it only change to 0 at the end of that epoch?

Once the transaction is executed at the destination (SC level) and the state change takes place, in the same moment the view function from the contract should represent the updated state / values (referring here to VM-Query direct to contract). One point here is if there is an intermediate API that has caching logic in it, that's where the value updates can get delayed