Some of the more minor wizards banded together and took over and made everything a lot more pleasant. Not only by putting down the major wizards but working spells to make the land work. And letting the non-wizards cast their magics, minor though they are.
Leading to a fair and pleasant and kinda steampunkish land. Hmm -- how long would it take?
How high was their tech level under the evil wizards? (Low. Even the most powerful wizards lived in conditions that the peasants of the fair and pleasant land do not endure.) How much does their magic help? (The steampunk is being developed mostly by those with minor magics, but the land itself is protected with great spells by the powerful wizards.)
At least a few centuries. Possibly more. Magic does have a tendency to cement their power, especially since they recruit lower-class children with powerful magic and even have a valid reason why they can't continue to live among their families, given their powers can be dangerous. . . .