The shadow appears the instant Zac leaves the ground and grows darker as he approaches. Below diamond, most people I use it on don't notice till I am about to land. In diamond, I am no longer surprised when a thresh flays me out of elastic slingshot or an alistar groundslams me out of it. It takes practice, and if you aren't hyper aware of that coming shadow, its easy to miss it. Hell, on the rare games I don't get Zac, it catches me off guard occasionally, but then again, I play tanks so I am not usually the target. Also, Zac's greatest weakness (just like fiddlesticks) is vision, so ward up!