Family Tree DNA states that an exact match (genetic distance zero) of the full-sequence test represents a 50/50 chance that the common ancestor lived within the last five generations, or about 125 years. To reach a 95% confidence level, that timeframe increases to 22 generations, or about 550 years.
While mtDNA mutates seldom and slowly, compounding the problem with using it for genealogical matching is that all its mutations are heteroplasmic. I'll spare you the biology  :-)  but suffice to say that you and your mother might be a genetic distance of one from each other, but her ancestors might be a genetic distance of zero from her for the past 2,000 years

GD) = Melitta Schneider (Bernard Siebert), Josepha Wand Thraen 1843-1911 (Luke Brudar Nassiff), Brock Aaron Brodersen and no line given.

Creatle thrulines You need 3 people who are no closer than 2nd cousin, and all must have the same common ancestor who is no further back then 4x, such as 4th gr grandma.  Also the names, spellings and dates for the ancestor must be recognizable to the software so it can tell they are the same person...and all on public tree.  For people whose ancestors are in the country 150 years or less there are often fewer circles.