When do we become Senior Airmen?

My friend and I joined the Air Force around the same time. I went in Dec 2006 and became an A1C when I left tech school because I enlisted for 6 years instead of 4. She went in the Air Force Jan 2007 but came in as an A1C and had 2 stripes in tech school. She enlisted for 4 years. When do we become SrA?