I don't mind long dramas if the story justifies it and its able to carry itself through all of its run. My longest drama so far was High Kick Through the Roof, and even though it ended horribly, I didn't mind watching so many episodes.
I've seen a few family dramas and they tend to drag when they reach the 40 or so episode mark. Of course that sometimes a story could be told in 16 episodes instead of 20 (or 28...), but all in all - I don't really have a limit as long as I'm entertained.