Ronald Reagan left the democrat party in 1962. He was elected president for the first time in 1980.
Donald Trump left the democrats behind about a year ago. If he is sincere in his conversion then he should be ready to be president in about 2032.
And to add to your point, Reagan left liberalism quite a while before he left the Dem party officially. He was out giving speeches about the evils of socialized medicine in 1960-61 if not sooner.