Quote Originally Posted by DarthEnder
It's my understanding that the Mangekyou takes a bit of your sight every time you use it.

If that's the case, I'm guessing Kakashi hasn't used it since his battle with Diedara.
Actually, according to Itachi your eyes "begin a slow descent into the darkness" the moment you achieve them so by that i'd take it that using them or not you'll get blind eventually

That said i'm sure that the more you use it the faster it happens