As long as it took the first time, probably? It's not as if this is a problem unique to Creationism. There will be one breeding pair at the head of any species under any explanation scientific or otherwise.
The carnivores might get pretty hungry waiting for enough deer to repopulate for a supply of food. Also, a “favorable” mutation is not necessarily a “one of” event. The change may well take some time of interbreeding to “take”. When examined, none of the worldwide flood propositions hold up. The proof of just what happened is literally in the ground you walk on.