If you want to change it on your server - you need to edit .\data\ skill_types.xml both on client and server.
Find <ability lvl="30" name="Uproot" id="35"> and change this string:
<duration const="20 10"></duration>
The first number(20) is basic Duration. The second number is level duration multiplier. Final duration is calculated using the formula:
finalDuration = basicDuration - skillLevel/levelDurationMultiplier.
For example with current numbers and level 60 final duration is equal:
20 - 60/10 = 14 seconds.