You should start with an Event Tick node. Connect it to a Gate node. The Gate node will allow execution to pass through while open (or while the button is pressed), and deny execution when closed (button released).
Create a Middle Mouse Button event. Drag off of the Pressed output, and connect it to the Gate’s Open input. Drag off the Released output, and connect it to the Gate’s Close input.
Drag off of the Gate’s Exit output, and create an Add Actor Local Rotation node. For the Delta Rotation input, drag off of it and make a Make Rot node. Enter the amount you want your actor to rotate per-tick. Pitch is rotating about the Y axis, Yaw is rotating about the Z axis, and Roll is rotating about the X axis.
Just a note: This blueprint relies on the Event Tick node, which means that the rotation of your actor will be dependent on the frame rate of your game. To remedy this, you can multiply your rotation by the Delta Seconds output of the Event Tick node. This will scale the amount of rotation per frame with the frame rate.