Honestly? I think it'd take long as it takes for somebody to start getting annoyed at Wizards constantly nuking their critters and do something about it.
As to what such a world would look like, I imagine the terrain would have very sharp, incredibly blatant demarcations between areas that have different "caretakers". As in "transition between zones in WoW" blatant. Likewise, each area would have whatever creatures suited the tastes of their protectors. For example:
-One area could be a dense forest filled with cute, fuzzy creatures that is defended by Druids, Fey, and the aforementioned fuzzy creatures that are much more vicious than they look.
-At one point, the trees would just stop right at the edge of a territory that a particularly powerful Dragon has staked out as his personal lunch box. Every last creature in the entire area is either "tasty", or too small for the Dragon to bother with. Any other creatures would be destroyed to make room (and reduce competition) for the aforementioned "tasty" animals.
-Just far enough from the Dragon's lair to not be worth his time, the terrain would change to the towns and farms of a Human kingdom, guarded by its army of mooks and even more powerful (if unofficial) army of Adventurers. All the animals would be what you would typically expect in a farm or town.
-Wedged between the Humans and Druids might be a Wizard's tower. Nobody there was strong enough to stop him from nuking what he wanted, so he set up shop and now keeps other Wizards from coming in and nuking what he sees as his living targets.