He returned home after six months at sea.

【翻译】他在海上过了六个月后回家了。

at sea，本句的意思是：“出海；在海上；海上生活”，作定语修饰 six months。那么，after six months at sea  意为：“海上生活六个月之后”。

又如：

After many months at sea, Columbus sighted land.  

在海上许多月以后, 哥伦布发现了陆地。

来自辞典例句

During his first few months at college David had been very conscious of his humble background.  

在入学以后的头几个月里，戴维深感自己出身卑贱。

来自辞典例句

My first few months at school were miserable.  

我刚上学的头几个月过得很悲惨。

来自口语例句