Updated and improved version HERE.
The idea here is for an action to repeat for as long as a key is held. These all achieve the same thing in slightly different ways.
USB Device Keys v1:
USB Device Keys v2:
If you don't like using Cancel all Macros (as in v1, above) and don't mind using two macros:
This one starts triggering and stops when a variable changes.
This one changes the variable on key release.