As far as I know, he will be away for 3 months.
用be away 这表示一个状态 与for相搭配
leave是瞬间动词 be away是延续性动词
As far as I know,he will leave for three months.
这是我自己弄得哦,比较符合当下英语的语言习惯:
he will leave in 3 months,as far as i know.
As far as I know , he will leave for 3 months.