I've seen people do ATL maps where the question is how many of the OTL borders do you keep.
Assuming a world where no single country controls more than 150% of OTL USSR, (to remove the Nazis rule everything case), how far back do you have to go before no land borders are the same as OTL?
As far as I can tell the oldest border currently existing is the Franco-Spanish border and that dates to the Treaty of the Pyrenees in 1659.