It takes roughly a year although some boys do tend to call a little earlier. Males will also eventually develop "nuptial pads" where their thumbs meet the palms. The girls will also get bigger than the boys but if you have a large male or a small female thats not really going to help you much. Give him time....if its a boy he will eventually start to croak for you. It also helps when you have multiple juvies..then you will start to see the physical differences in them as they get older together and you'll have something to compare them to.