To get to Planina Zapleč, go from Krn to Krnska Škrbina and then a few minutes toward Krnsko Lake. Soon a path branches off to Pl. Zapleč and Vrata, which is marked to Pl. Zapleč. Then along the road or across the pastures. The path across the pastures is very nice but more or less overgrown (depending on how much the cows have grazed). Until last year, there were only occasional faded marks here and there, so you have to follow cow paths and a bit by feel. The pastures are crossed by two gravel roads or tracks; on the second there is a sign directing to Drežnica and Curk waterfall. The branch is also marked from the road to Drežniške Ravne. I don't know the path and don't know how the next junctions are marked.
For the path from Pl. Zapleč to the trough and parking above Drežniške Ravne across pastures, we might need maybe a minute less than by road, which drags on but allows faster steps (around 45 min). From the parking above Drežniške Ravne to Drežnica it's approximately the same. Pl. Zapleč-Drežnica: around 1.5 hrs.
The path from the parking above Drežniške Ravne to Krn is marked at 5 hrs. I myself went only to Krnčica. To the valley thus at least an hour less, unless you're like me, who needs almost more for the descent than the ascent.
I'd like to provide more accurate information, but unfortunately I haven't managed to fully explore the Drežnica-Krn path yet. I'm most interested in the path to Drežnica past Curk waterfall, which will remain on the wishlist for a long time. Thus, I can only help with half-verified information.
Description of the path Drežniške Ravne-Lopatnik (path to Krnčica and Krn):
http://www.hribi.net/gora/lopatnik/1/2309