Have question regarding my parents US visa application, I am greencard holder
The last time my parents applied visa was
签证日期： 21 DEC 2011 签证到期：19 DEC 2012
第一次入境： 8 FEB 2012 第一次出境：6 AUG 2012
第二次入境： 6 OCT 2012 第二次出境：4 ARP 2013
They are planning to come to US again in Aug 2013, and is preparing to to apply B2 visit Mid July. 7月中旬办理B2签证（免面谈）
Becuase this is only 3 month after they come back from US, is there any risk that the visa application will be rejected.
If there any rule about how frequently can you apply US visa?
Thank you very much for your help