Once a week I like to trade 10-20 kicks to my inner and outer leg. The last couple are hard to take. When I first started, I could barely handle a tap. After a while, I could take 50%+ shots for the whole set from someone my size.
I don't know if you can take full power shots right on the sweet spot, but in sparring you are moving around a lot and you will get good enough to be out of the way. In a real fight, you have adrenaline on your side.
Two separate things I've been chewing on for awhile is if the trading of kicks is really necessary for martial arts. Absolutely, I have no doubt that doing the drill trained me to take harder kicks cold when I'm not feeling any adrenaline. This lets me spar harder than an average martial artist which means I get more good practice in.
On the other hand, in a real fight you will have a reduced feeling of pain. The worst hits I've taken in my life, falls, car accidents, and so on I didn't feel anything. If that were the case in a fight, the conditioning might not be necessary.
I never had a sense that I was making my leg any different by kicking. I always thought that I was teaching my nerves to understand that I wasn't taking real damage, and so they won't spaz out. That's why real damage, like a full power kick, still can wreck me.
Finally, even without trading kicks, sparring a couple rounds a week for a couple years might toughen you up either way, so long as you are taking meaningful shots. I've known very tough men who got that way through sparring, and I've known really soft men who were never pushed sparring and were afraid to be kicked.
Martial arts are an art. I don't know if anyone knows what the best way is. Cultures where people start kicking one another in the legs as kids might be different because they started getting hit so young they may have a perspective on conditioning that isn't useful to the serious adult or weekend warrior. You could always watch some training videos of Muay Thai or Kyokushin and see if you can spot people trading hits. I am sure you will find examples of it from the professionals.