If we accept the premise of this question, which is that Christopher Columbus “discovered” America, then the correct answer is that he did so in 1492.
There are many ways in which to argue that Columbus did not really discover America. The Native Americans were already in America, so clearly they had “discovered” it first. The Vikings had definitely reached the shores of North America centuries before Columbus did. Columbus himself never knew or believed that he had discovered a new continent. He believed that he had reached islands off the coast of Asia. This means you can argue that he himself never discovered the Americas.
However, it is conventional to say that Columbus discovered America as he was the leader of the first European expedition to reach the Americas and to establish a permanent presence there. If this constitutes “discovering” America, Columbus did it in 1492.