Yes, an important point is that mutations happen slowly. So slowly that effectively, there is no noticeable difference in the DNA samples taken from a person over their lifetime. IANABiologist, but I think the process of DNA testing goes like this:
There are certain known points on each of our chromosomes, common to almost everyone. To create a DNA “fingerprint”, in very basic words they treat the sample with chemicals that “snip” the DNA into segments at known points. the snipped DNA is put on one edge of gel on a film and a small voltage is applied across the gel. This voltage causes the DNA samples to migrate - the bigger they are, the more junk segments in that active segment, the slower they migrate. So for certain known segments of DNA, they get a weight ( equals length) number. Given a few dozen random numbers, the odds that any two people match within any accuracy is astronomical. (Unless they both got that DNA from a common ancestor)
This is the DNA test used to identify relatives, criminals, etc. Even then, the change is slow enough that for example, to solve the question of whether Thomas Jefferson had children with his slave, Sally Hemmings - the DNA test between descendants on both sides showed a connection. However, the test could not distinguish between Jefferson and his uncle or brother as the father. (IIRC they looked for direct male descendants and compared the Y-Chromosome, since it would be the one passed directly down.) Similarly, I recall reading about how some body of a man found ceremonially killed and buried in a bog in England several thousand years ago was preserved enough that they matched his DNA to people still living in the area.
Also note that a lot of DNA changes are to “junk” DNA, segments of DNA between the active parts that appear to serve no function - so changes to them do not hurt the body functions. Similarly, changes to chunks of the DNA which actually make the cells work and produce results - usually are not good for the cell or the body - think “cancer”. But generally there’ a feedback mechanism where significantly changed DNA will cause that cell to die rather than split. Plus, and changes that occur randomly in some odd part of the body don’t migrate to other parts of the body, so each location’s DNA likely diverges a bit from the original blueprint.
So to answer the OP’s question… so long that it’s unlikely to be relevant.
This is not the same as analyzing every segment of your DNA small bit by small bit looking for similarities. Presumably, doing this one can find some changes.