Longest Kdrama you watched

High Kick! The Revenge of the Short Legged 123 eps.

Five Children (54 ep)

Six Flying Dragons

For Korean.... its Six Flying Dragons - 50 Episodes

King's Daughter Soo Baek Hyang, 108 episodes.

However, since those are 1/2 hour episodes, the actual longest series I've watched in terms of length is Queen Seon Deok, though not by a whole lot.

Six Flying Dragons 50 Eps 

Ugly Alert, 133 episodes :)

Stairway to Heaven, 121 eps