You can continually zoom using the Zoom Realtime command (View | Zoom | Realtime). When Zoom Realtime is activated, holding the left mouse button down and moving the mouse up will zoom continuously in, and holding the left mouse button down and moving the mouse down will zoom continuously out.
You can also scroll the mouse wheel to zoom in/out (scroll away from you to zoom in, scroll towards you to zoom out).
There is not a way to customize the increments with the Zoom In and Zoom Out commands.
Updated: October 6, 2016