六年 + 来/以来
The noun phrase 六年 (liù nián) means 'six years.' It specifies a duration of time equal to six years. In Chinese, time durations are formed by placing the number directly before the time unit, with no additional words needed.
他在国外生活了六年才回来。
Tā zài guówài shēnghuó le liù nián cái huílái.
He lived abroad for six years before coming back.
这个项目已经进行了六年了。
Zhège xiàngmù yǐjīng jìnxíng le liù nián le.
This project has been going on for six years already.
六年的时间过得真快啊。
Liù nián de shíjiān guò de zhēn kuài a.
Six years have really gone by so fast.