Well, there are many things to weigh in regards to cost-benefit of buff spells like Protect & Shell.

Positive:
  • Effectiveness
  • Duration

Negative:
  • AP Cost
  • MP Cost
  • Casting Time
  • Recasting Time

I am confident that the Development Team can find a way to increase effectiveness and duration, by counterbalancing AP cost, MP cost, cast time, and recast time.

Personally, I prefer Duration and Recast Time to be as close to equal as possible. It is also my personal preference that buffing spells have generally a longer casting time than curing, direct damage, or debuffing. Something around 5 or 6 seconds. Nothing too severe though (SE please don't triple casting times). Just enough so that the mage has to make a tactical choice on either rebuffing mid-fight, or not doing so in order to heal or DD. Naturally, the effectiveness of Protect & Shell should be made potent enough to warrant considering such a tactic.