Not long. Like I've said, I'm not original. I just found a map of ancient Rome, cleaned it up, and added my own twist to it. You can still recognise some areas on the modern map of Rome. Same with the port, which is based on the Roman port of Ostia.
Even now I'm still adding things as I just realised I've no prostitution area (red light district) in a port of all places, plus no granary for imported wheat. Added a bunch of fresh-water reservoirs and firewatch stations as well(Hastily beefed up in the chaotic year of emperor Ablag 150 years ago, to try and win popularity from the mob after the infamous emperor Oren was assassinated by panicking senators during a mob revolt when rumours circulated that he played the flute during a lavish dinner party while Imperial City burned).
(Literally just went with the flow and "made" that last bit up just now, and am going to copy-paste it into my lore notes for the history of Imperial City. Can you guess what this highly original bit of lore is based on?) God I love world-building!
Currently reading Robert Grave's very interesting interpretation of Greek myths. Will probably use that as the basis of Eleven society in this world. Still a lot to tinker with.