Anywhere from 13-20 weeks but to get the most accurate answer will be given around 18 weeks.
It depends usually on the place, but the one I went to can tell starting at 14 weeks, don't know how accurate but I felt like I was having a girl at 7 weeks, I just felt it, and when I went for my scan they told me a girl too :D I will find out for sure tuesday dec 3rd