From when the customer triggers the process to when the customer decides or is satisfied that the process is complete.
This means not when the provider thinks the process started or when provider thinks the process is finished. e.g. The call may complete from the process perspective but the customer may not have his/or her problem fulfilled. When the reason or problem is not closed the end has not been reached.
Many companies publish self congratulatory SLA statistics that evidence to them good service but the clients in reality think the company is "rubbish".