Using a good grade of US made 12 ga galvanized as your center hot wire I suspect that it will last as long as I do.  I would be overjoyed to live long enough to have to replace that wire!   I think all fences do need attention every so often. the better job done designing and installing them the less maintenance they will need, but they will need some maintenance at some time.

Ben, what do you like for a perimeter fence (or pen) fence?

Jim at Dawn