For some reason I can't access the leaderboard. Maybe because I'm not doing the challenge this time, but bear in mind that his 100 lessons could be based on 30 minutes/ lesson, so that makes them 50 hours so far, which is also a lot!
During past challenges I came across people doing upto 350 lessons and back then the challenge was just for 30 days, and not like now for about 45 days.
Now Nomayne, Speaking from a personal experience, I used to take/give 3 hours lessons a day sometimes. Of course there are people who deal with language learning as the most important aspect in their lives, so they could be doing way more and 100 lessons/hours for them in 4 weeks (considering that this challenge started on Apr.9th) is just normal.