how long should B6 Bilsteins last?

There is an age factor that leads to less than optimal performance as well, a shock does not have to leak to under perform. It appears you have replaced them and they are not the original stock shocks, but if they are over a dozen years old you would see an improvement with a new set. About the only way to truly find out is to remove them and test against new, shocks do not perform as well as new when they have considerable mileage on them.

I believe that the B6 Bilsteins have a lifetime warranty to the original purchaser so just keep your paperwork and go out and put on some miles.

100,000 miles