I don't think it's a certain test or a certain amount of time to be considered in remission. After tests, scans, blood work, etc. come back clear for a while, people say they are in. My mom was diagnosed with breast cancer 13 years ago. I faintly remember her saying she was in remission after her chemo and radiation, but I was young and shielded from real world stuff back then. It spread to her lungs in 2004. I remember her saying, "I was almost in my 5th year of remission!" but I don't know if her oncologist actually told her that she had been in remission, or if it was just her way of explaining how long she had been cancer free.
I'm surprised your doctor didn't go into further detail after asking her this. I really hope she didn't just say, "No, there's no test for that!" and leave it at that. I can only imagine what having cancer would do to a person mentally, always wondering if it's still there or if it's going to spread. Normally they run tests for a period of time after initial treatment, and let you know that there are no signs the cancer is still there or it coming back, for reassurance. Best of luck.
Add--> I wanted to add that my mother is still here and fighting. I made it sound as if she had passed. Since 1998 it has spread to her lungs, bone, liver and colon with a 2nd diagnoses of breast cancer in between. But she keeps fighting, working full time and living an otherwise normal life.
I know it must be frustrating to never hear "you're in remission" but that's not what matters. You're here, you're a survivor and you've kicked cancer's butt. :)