They started out at a 290 ESR barrel life, by the late 60s deployments developments in powder and even the chemistry of the powder bags greatly extended that. During the 1980s deployments, they were up to around a 1500 round equivalent life, and replacement was necessitated more by damage to the actual barrel structure than the inside of the bore, which was the original limiting factor.
There wasn't anything efficient about the Iowas, but they were great terror weapons and really, really good at pounding through heavily-reinforced concrete. I think the 2,700lb. Mk. 8 shell was capable of blowing through something like 30ft. of reinforced concrete. They were recognized as obsolete before even the four that were completed were commissioned, but construction was allowed to continue because they were fast enough to serve as really, really big escorts for the Essex-class carriers. The Montana-class was cancelled before any construction began because they were too slow for modern naval warfare, and the resources were needed for carrier construction.