A longer duration would make the skill more "usable", yes, but is a longer duration needed? The skill is not unusable in its current state, nor is WHM reallt lacking healing power or flexibility imo. I feel like an increase to the duration would cause WHM to dance the line of OP and balanced since being able to hold onto a PI for a long while would not only make them more flexible, but also allow them to have something additional that could help them save MP (even if it's a long term investment).