Start with any of the multipart series, whatever interests you.
Punic Nightmare (mostly about the second Punic war) 3ish hours
Ghosts of the Ostfront (Eastrenfront WW2) 5ish hours
Death Throes of the Republic (last 100 years of the republic till the death of Julius) 10ish hours
Wrath of the Khans (Mongolian empire)
Blue Prints for Armgeddon (WW1) 16 hours and not finished yet
1 off episodes that are really good Thors Angels (how the barbarians that took over Rome were not really 'barbarians' but semi-romanized Christians to a good extent), Phophets of Doom, American Peril (about how America got is empire).
They get longer and better the newer they are. Each of these are about 4 hours.