If you finish your masters before starting the PhD, the time to complete your PhD can be shorter. You can look up the statistics (don't care enough to find them myself), but the average length of a PhD in physics is 6 years. As pqor mentioned, you can do it in as little as 4 if you work really hard. However, if you already have your masters degree, some PhD programs will let you place out of some of the courses you have to take. For example, here at Ohio State, I know someone who came in with a masters. The department let him place out of all of the "core courses" (EM, QM, Classical, SM/Thermo) except EM (the prof refused to let him place out of it). So he's nearly done with all his course requirements in his first year. That means he can start focusing on research much earlier than those of us who still have to take ALL our courses. So if your department lets you place out of some courses early on, the average time to complete the PhD is probably lower. This is something that varies by department.