Hmm, I was thinking 30 seconds every long while, but if I had to put a number on it. Every hour? Cause the power is supposed to make my character mentally tired (Forgot to add that part). So, if I do use the whole 30 seconds, then my character would be mentally exhausted/sluggish as if he just took a 3 hours standardized test. Normally I would probably use it for a few seconds. Also, how about if I expand the iron part to any solid metal? The power is not really meant to be offensive. Mostly defensive/support. The offensive part that really comes with the power would be the fact that I would probably be able to surprise someone who wasn't expecting it. I mean... imagine someone just randomly appearing behind you. That and the fact that in places where its not bright it would be hard to see me, so I can sneak around a bit.

Edit: Course, if you want me to come up with a different power, then I can. Hmm, powers that are similar to the freedom theme...