回複：AP上有有效期，通常一年。你必需要在該日期之前回來，次數不限。

The 回美證 refered to above post means Re-enty Permit (RP), not AP (RP application procedure is the same as the AP). The reentry permit is for green card holder to stay outside U.S. > one year, and its有效期 usually for two years.