It depends on the school- some don't really pre screen aka don't have a hard set minimum gpa so you may get one in a week or 6, it depends on the number of apps they have to sift through. Others may take longer considering you- there's not really a set time. Some have automatic secondaries, but I think a lot of those are MD.
By pre writing, I assume you're going off old secondaries? I mean, I'm not pre writing mine personally- I'm a re applicant so I do have all my old Q and As but it personally doesn't take me more than a few hours of hard thought to put together my thoughts coherently. I will have to rewrite them again, and questions may change. If you feel the need to pre write and submit within an hour of receiving it, by all means do so, it still ends up with the other stacks of secondaries.
Whatever strategy you need to do, you go ahead and put those proverbial balls to the walls.
