I really don't have any idea, but I am assuming that it is until they are big enough to not be able to fit through the bars. And then maybe a little longer to be sure!
I have little ones that are just over a week old. I have 10 inch cardboard tucked behind the coroplast. It was really easy to do, and it isn't the worst looking now that I have all one color cardboard in there now. Before I had cardboard from my recycling and it was buggin me! LOL
Usually by the time they are 3-4 months old they don't need the cage babyproofed anymore. They have to be large enough not to be able to get their head through the bars.
Well since pigs vary in weight and size even if they are the same age, what would you think is the right weight to no longer need baby proofing? My baby is almost 3 weeks, and almost 300G. Mom is like 630G and i think 4 months now... Bean is 509G and around the same age. Neither seem like they could get stuck even if they tried. So would you say over 500G doesn't need baby proofing anymore?