何时可以省略时间状语从句的介词，何时不可省略？

又如这句，Three years. at least four failed study designs and countless hours of insignificant analyses later the team found what it had been waiting for. 

这句话的three years 和 at least four failed study designs and countless hours of insignificant analyses 是什么关系？同位语？

如果去掉three years，那么At least four failed study designs and countless hours of insignificant analyses later the team found what it had been waiting for. 这句话对么，看着好别扭。