This website says 48 month:
http://ustraveldocs.com/cn_zh/cn-niv-visarenew.asp
您以前的美国签证在到期后48个月之内进行续签。
But when I try to arrange 申请免面谈 from the following site:
https://cgifederal.secure.force.com/?language=Chinese%20(Simplified)&country=China
It gives a list of questions to see if you're qualified for 免面谈, one of the questions asks if your visa has expired within the last 12 months.
I'm totally confused: 12 months, or 36 months or 48 ?
Thanks ahead