Your mtDNA would be identical to your mother's maternal line back as far as you are likely to trace the many generations back.
My earliest mtDNA was theoretically traced back to a Mary Clark, 13 generations back, but for a long time I was not sure of the accuracy of my research over that many generations. Someone else with the same mtDNA also traced their line back to Mary Clark, so at that point, I felt it was likely to be a valid DNA connection.