Currently the only way to change the “insert length” is by assigning a key command to a value:
1/16 etc etc
You can assign hotkeys like this for quantize amount, but you have the added funcionality of being able to scroll through the values using 1 or 2 hotkeys. With insert length this feature is omitted. The inconvenience is that you are required to assign so many hotkeys. It would be easier to just scroll through them with 1 or 2 hot keys.