Four generations is 100 years. Brain is still a teenager and Eraqus is his grandson (presumably), meaning the Dark Road era is probably around 50 years later, so 150-ish years after the time of Ephemer. 75 years after that is KH3 era, so 225 years.
The only real question is when in relation to Daybreak’s destruction did Ephemer arrive? It still leaves room for interpretation for how long after the events of Chi and Union Cross that happened.
EDIT: Actually wait, the wording of the translation is four generations “after Ephemer was alive.” So if we presume that he lived a long life building Scala and recruiting Keyblade wielders, then the 100 years would be even longer. So if Ephemer lived to 75-80 years old, that adds another roughly 65 years to the tally if Ephemer is 15-16 during Union Cross, making it around 290 years since he arrived in the future. So all in all, it’s looking like the timeline between Scala’s founding and present day is close to 300 years.