It's hard to say, to be honest. There are some barefoot groups online with an almost fanatic devotion to barefooting (up to considering shoes a serious cultural evil) and a lot of them claim that you can reach that point, but that it takes a very, very long time.
104 days wasn't long enough. The way I'd put it, you never get used to it - you get more used to it. Toward the end of my journey I was zooming over ground that would've crippled me earlier on, but I still wasn't able to walk over chunky gravel without grimacing. One of the hardest bits for me was near the end in Glenveagh National Park in Donegal - I had been starting to think at that point that my feet could walk over anything, and then I was humbled by the road.
Getting used to barefooting in urban areas is easy, though, because the pavements and roads are usually so smooth. If someone thinks their feet are very tough but they've only been walking around the city, they're in for an unpleasant surprise outside the concrete jungle.
There's also the matter of attrition - when your feet are fresh at the start of the day it's easier to walk over rough ground, but soreness will develop as the day goes on. It's generally variable though in that there's no proper linear progression in toughness over time, and terrain that was fine two days ago can be difficult today.
I think I'm going off on a tangent. If you wore shoes growing up, can your feet be retrained? Probably there are two elements. On the physiological side, the soles of your feet will need to toughen dramatically, and your feet will be misshapen from wearing shoes (crushed toes and narrow feet). This in turn will affect your gait, so it's unlikely that you'll walk in a way that minimises pain. Probably it takes much longer for the shape of your feet to go back to "normal" - if ever - than it does for the skin to toughen up. I think my feet widened slight and my toes came apart a little, but not nearly like a lifelong unshod foot.
Then there's the neurological side of things. I'm not a neuroscientist, but this is how I've had it described to me. Your feet are quite sensitive and send a lot of information to the brain, but because they're trapped in shoes, they essentially can't feel anything. When you take off your shoes, your feet are suddenly exposed to a huge amount of sensory input, which is incredibly uncomfortable, in addition to the pain of things poking into your feet. So I imagine it would also take some time for this to diminish.
It's pretty clear from peoples around the world that if raised barefoot from childhood, people can comfortably walk over ground that would make most of us cry. It's hard to know if this level can be fully re-attained.
There's a bit of a caveat though in that it depends on the terrain. I mean, there has to be a natural limit. The Tarahumara in Mexico are often used as the shining example of barefoot runners (thanks to Born to Run) but they wear sandals because where they live is covered with spiny plants and sharp rocks - no-one is walking over that barefoot.
I'd say if you have kids, try to have them walk around barefoot as much as possible. It'll probably make easier them to go unshod in future, but more importantly it's apparently better for your feet and legs and by extension your back.