Actually the Black Cero notably has a longer charge time than a regular cero, as shown by the fact that Ichinator can go from zero to cero while Ulquiorra was actively charging it. Guestimating using dialogue given as the orb was charging as well as that fact, the charge time for a Black Cero should be about 2 seconds or so before it fires at best given all three times he used it. Hell, keep in mind Ichigo can fire off and fling a Getsuuga all the way to Yammi in the time it takes to charge the damn thing.
Make no mistake, cero's don't take any absurd amount of time themselves, a couple of seconds at most usually, but in a world with instantaneous movement, a second is literally nothing. The second you see the orb formed, you should already be out of the way.
Notably the reason I say Cero's don't appear to be movable is multifold. Even taking into account those given factors, the point of orgin is always somehow key. Whenever that's shown to be disrupted, be it cutting off a horn that presumably it was charging it from, striking the orb with a fist or an energy attack, or hell kicking the dude charging it with his mouth in the face, the cero has always either destabilized or outright blown up in their face.
Any disruption of the point of origin during the charge appears to outright negate the cero in some fashion. That plus the fact that a cero has never been directed even in a world with instant movement leads me to the natural conclusion that once the charge starts, you're locked in a particular direction for firing.