It means that the writer of the song has been waiting all his life for the perfect girl to fall in love with, hence the "
I've waited a hundred years" part. The next part "
But I'd wait a million more for you" signifies his true, passionate love for a woman and he would wait for as long as it takes for her to realize her true feelings for him too. in conclusion, it just means that he has fallen in live with this lady and would wait forever for her to be his.