I would also love a duration increase but for other reasons.
We have already a lot of skills to use during battles. The need to cast Protect every 5min seems nothing more than a pointless nuisance. I don't even bother using it, when i solo. Effect is low, duration is short.

Would be different if Protect would be very effective, but Protect I isn't.
Maybe increase Protect I or bind the duration with MND.

With Rank 15 Protect I is effective, but don't last long.
With Rank 50 Protect I, would be weaker, but would last very long. Protect II would be stronger, but shorter.