Depending on where you travel (distance), 20 roundtrips a year could net you Gold or higher.
It's all based on the calendar year. Once you reach either 30 segments or 25,000 miles flown (actual BIS 'butt in seat' miles, not including bonuses) your account will switch to Silver status. If before the end of that same calendar year, you reach 60 segments or 50,000 BIS miles, you'll change to Gold Preferred.
See the US Airways website for more detail with special attention to the TRIAL PREFERRED section. If you know you'll be flying quite a bit, it may be worth your while to sign up for (and pay) for a Trial:
http://www.usairways.com/en-US/divid...toqualify.html