Feathers Regrowth time?

So when I bought my mallards back in February the seller clipped their wings cause they would fly around the land and swoop all over. He said it would be best so they can get to know the new area and the get familiar with house. Now my question is,

how long does it take mallards to regrow their feathers? To this day they cant but try daily and run from the top of the hill. With no luck.

I read its a yearly time frame but am afraid the seller might have cut too deep