"The Forever War" was my first introduction to this form of time stretching. I actively search out authors who incorporate it into their storytelling, so I was thrilled when I stumbled across Alastair Reynolds' writing.
I think it lasted indefinitely. Both people had life-extension technology on their ships and they were Bussard ramjets of the "can accelerate forever" variety.
Another classic relativistic story is Pohl's _Starburst_.