On FamilyTreeDNA, for haplogroup H1, my closest matches are all genetic distance of 2 for HVR1, HVR2, CODING REGIONS. I don't see any tool to predict the number of generations like we have for yDNA test, so I'm wondering what is the likely number of generations given that mtDNA mutates more slowly than yDNA?
Mitochondrial DNA mutates very slowly. Like on the order of one mutation in a thousand or so years.
FamilyTreeDNA have published a guide on Understanding Your mtDNA Full Sequence Results. For the Full Sequence mtDNA test you have done, with a match genetic distance of 0 (i.e. a perfect match) you can be 95% confident that the common maternal ancestor occurred in the last 22 generations (~550 years), and 50% confident it occurred in the last 5 generations (~125 years). You can see this is a very broad range.
A mtDNA match with genetic distance of 2 is likely to have a common maternal ancestor several hundreds to thousands of years ago. As far as I am aware there are no reliable tools available to estimate how many generations back was the common ancestor of a mtDNA match with genetic distance >1, simply because the range is so broad as to not be useful. A perfect match could be your mother, or your sixth cousin. A match with 2 mutations could be your fifth cousin or your 40th cousin.
An early Full Sequence mtDNA report gives the following data for genetic distance 2 match:
- 50% probability in last 18 generations
- 75% probability in last 26 generations
- 90% probability in last 35 generations
I am not sure of their source for this data, and I suspect it is probably based on a small sample size. I would treat these estimates with extreme caution.
This is why mtDNA is not hugely useful in the genealogical timeframe. It is, however, very useful in disproving relationships. If two people who supposedly share direct maternal ancestry do not have a close or perfect mtDNA match, then you can be very confident there is a mistake somewhere in your documented family trees.
In summary, you can be confident you share ancient and distant maternal ancestry with these matches of genetic distance 2, but it is probably so distant that you will never possibly work out what the exact relationship is.