On average how many units will be produced until the control chart signals that the process is out of control when it is under control?

A firm that manufactures notebook computers uses statistical process control to monitor all its production processes. For one component, the company draws samples of size 10 every 30 minutes. The company makes 4,000 of these components per hour. The control limits of the x chart are set at 3 standard errors from the mean. When the process goes out of control, it usually shifts the mean by .75 standard deviation.