There is no test that can tell you how long you've had it. Many people can roughly narrow down the approximate point of infection, but many never know. A biopsy can reveal the extent of damage to the liver and is important in deciding what to do about hcv (to treat or not). But, liver damage rate of progression is different in people too. A person who has had hcv for 10 years can have damage equal to a person who has had it for 30. In the end, it doesn't matter much. The important thing is your current health and other factors in your life and assessing what you will do about hcv, if anything.
FL guy is right.
It's not a magic number - it's more that most likely they can guess due to how much damage we have AND what our lifestyle was like. But there is no way to really know by that.
For example you could have had a transfusion or used IV drugs in '88 but then gone to a dentist or nail salon in '93 and there would be no way to know which gave it to you.
The amount of liver damage that we have is the important thing to know when deciding whether or not we NEED to treat. Personally I decided I was going to even before I found out that I was already at stage 3 liver damage - I had no symptoms that I know of AT ALL so I consider that I was very lucky to find out so I could make sure to quit drinking and try to live the most liver healthy way I could. I can't imagine how much damage I would have done if I didn't know.
That is why they call it The Silent Killer.
Good luck!
My thought, too, is if there isn't alcoholism or some other chemical reason why the liver enzymes rise and the biopsies show damage in addition to Hep C, the docs attribute the liver damage solely to the Hep C and for some reason they ascribe the magic "30 years" to how long it takes to damage your liver with Hep C. My H was pretty sure he got Hep C from his tatoo and the time on that was like 26 years...son of a gun if the family doc didn't say "you've probably had this 30 years" to my husband after he was diagnosed after his liver enzymes elevated slightly during a routine annual physical.