I would say a couple miles for me..... It just feels more natural and less resitance from unnatural things.......
Whilst I have only been keeping track using a hiking app for the last 5-6 months I have gone extended periods barefoot throughout my life often walking all over the place! At the moment I am usually going for a walk of between 6 and 15 kms every weekend. It's usually a combination of concrete/bitumen and grass and sometimes sandy riverfront. At the end of the walks my feet feel great and, as you say, it is very natural I prefer a mix of surfaces as I like feeling the different surfaces on my bare soles. The harder surfaces are also great for toughening soles. I do the whole walk barefoot but do carry a pair of sandals in my small backpack just in case something goes wrong (weather gets too hot to go barefoot or I cut my foot - nether happened yet thankfully!)
I have walked 12 miles barefoot on a trail that has gravel dirt grass mulch sand and it all felt so awesome and I will be doing that this weekend.
I don't know how many miles it amounted to, but in July 2014 I did a two weeks stretch of walking barefoot only, not wearing any footwear whatsoever, because the summer that year was fantastic. Most every type of surface was encountered, except for lava and hot coals, and yes, I walked a lot.
I'm...not sure in terms of distance. But I've gone on a few day hikes in Maine that took most of the day. I didn't bring footwear.
My kind of Roads I love to walk on barefoot and my kind of mountains vegetation I love to walk-through