Zooming should occur instantaneously, so that seeing the content change size would give the necessary feedback. Zoom indicator is not needed in many cases.
However, an indicator can be provided when it helps the user understand the current zoom level among the available levels. For instance, if there are three text sizes and the largest is already in use, it is helpful to see an indicator when trying to zoom in. The indicator should be a floating component and disappear after a short time-out.