Here's what it does, from what I can work out (seeing as Microsoft support uselessly repeated the unclear text in the options window):
'Speed' - the first slider - is how quickly right-clicking activates when you hold the pen down.
'Duration' is how long you can hold the pen down before right-clicking is cancelled. ie, a short duration will mean that once the right-click has 'activated', you must lift the pen quickly to properly right-click. Otherwise nothing will happen. A long duration will give you more time to lift the pen, and still right click.
A short duration is useful if you find yourself changing your mind - if you accidentally right-click, you can hold the pen down for a bit longer, then release it and there will be no right-click.
A long duration is useful if you're a bit slower with the pen - you hold it down to right click, then you have plenty of time to lift it again without cancelling the right click.
I hope that makes more sense.