[Help] 延续非延续动词的疑问

My brother has been an armyman for ten years.

这句话想说想成为一个军人。而has been后不能跟for加一段时间。

To me it sounds vaguely correct too... The only other possibility coud be: 'My brother has been an armyman of ten years.'

... Still, it doesn't sound right either. Perhaps the reason why it's wrong is because there's no such word as 'armyman'...? It ought to be a soldier instead of armyman...?