How long does a foreign woman have to stay married to a US man before she can divorce and get her "papers"?

My girlfriend is from the Czech Republic and she married a man from the United States and she is still married to him, but no longer loves him. She said she wants to marry me instead. The thing is that she has been in the US for less less a year and we are afraid that if she divorces him to marry me she will get deported. How long will she have to wait before she can legitimately divorce her current husband and marry me, yet still stay in the United States legally? Any serious responses would be greatly appreciated.