ten years of hard work：

按结构理解：ten years 是中心词；of hard work 是修饰语。即：ten years (of hard work)。

按意义理解：hard work 是中心词；ten years of 是修饰语。即：(ten years of) hard work。

英语中这类表达十分常见，例如：

Ten years of steady economic growth made the country stronger.  

10年的经济稳定增长使国家更加强大。

He has served five years of his sentence.  

他已服刑五年。

来自《现代英汉综合大词典》

After six months of writing, his report is a powerful response to his critics' mistrust of his earlier research.

经过六个月的写作，他的这份报告针对他的批评者对他早期研究的不信任，是一个强有力的回应。

After ten years of marriage they decided to separate.  

他们婚后十年决定分居了。

来自辞典例句

I profit more from one consultation with you than from ten years of reading.  

听君一席话,胜读十年书。

来自《现代汉英综合大词典》

同样一个词组，侧重结构还是意义，在不同的语境中要作出具体判断。例如：three cups of tea。红色的为中心词：

There are three cups of tea on the table. 

桌子上有三杯茶。

I drank three cups of tea.

我喝了三杯茶。