Spells: Druid : Shield of Barbs

[Twig]

So yesterday I dinged 19 with my Druid and after buying my new spells, went back fighting Gnolls in EK.
After loading up with buffs to beating up on gnolls I casted SoB and was gladly surprised to see a duration of 6 minutes and I was 'Woa that new one really lasts longer' and I managed to melee 3 Gnolls before it faded out.

Today I log on and find out SoB lasts 1:54 so basically 1 fight, maybe 2 if I'm lucky. So I'm wondering how come it lasted 6 minutes at once ? Was it first use only ?