Xiaobaitu and others,when parents 2nd enter US with 1 year multi

My parents has been here for 5 months with multiple entrance B2 visa. They will be back to China soon. We planed to let them back to US again after 3 moonths to take care of baby. I understand they come back with out applying visa within 1 year. Right? My question is when is the best time for them come back again. I read one of your post said you have the experience. Can you give more details like how long they stay in China before their 2nd entrance to USA?Thanks a lot!