Not bad, to be honest. Considering you can get only one swift action (which are the majority of the magus's other arcana abilities) a round OR an immediate action, but not both.
It has some merit, and I'm half-tempted to allow the magus in my game to use that for one of her arcana, if she wants to.
And then maybe have a two or three Sudden Shield tree:
Improved Sudden Shield: When using Sudden Shield, the magus can increase the duration of Sudden Shield to a number of rounds equal to one-half his magus level by expending an extra point from his arcane pool (for a total of three).
Prerequisites: Sudden Shield, Magus Arcana; Magus Level 9
Greater Sudden Shield: When using Improved Sudden, the magus can split his rounds of durations to all allies within 30 feet by expending an extra arcane point from his arcane point (for a total of four). The magus must be able to see each of his allies he wishes to grant his bonus too and the rounds must be split in one round incriments.
Prerequisits: Improved Sudden Shield, Magus Arcana; Magus level 15
Again, I like the concept. Go for it.