Board Thread:Naruto Discussions/@comment-24824148-20160724215526/@comment-734582-20160725101831

AsianReaper wrote: I never actually thought that at that moment he was experiencing it. But now that you say it, it makes total sense.

Judging from the aftermath of Sasuke V Bee, it seems you experience blurry vision after Mangekyou use, even when that MS is new. So basically, you'll quickly notice the effects of the MS. Kakashi was already familiar with his eye's power when he used it against Deidara, so he clearly had been working on it for some time.