The rock wool should last, basically indefinitely, given it doesn't get knocked around. (Not likely in your use.) There's a rock wool panel on the inside of the feed door on my furnace. I replaced it shortly after we bought the house, the previous owner had partially ripped it off. So far going on 14 years and zero degradation of it. I did use a ridgedizer on it to stiffen up the face thay sees the flames, but not needed in your application. You should be good to go for a very long time. Happy it worked well for you.
