Short answer that leaves much unsaid: Never.
Long answer: I intended to do proper cityscapes when I first took up 6mm sci-fi, especially with the Old Crow buildings. Some ten years back I even had grand plans for modelling a specific city with a kind of Latin American, African and South and Southeast Asian flair for a homebrew Cowboy Bebop-inspired setting where Venus had been ultra-rapidly terraformed and settled mainly by people from third world / emerging market / newly industrialised countries. Oh, it would have been glorious!
…but then I became increasingly unhappy with the dimensions of nearly all urban-type 6mm sci-fi buildings on the market. They just don’t look to me like they could accommodate reasonably-sized apartments, offices, commercial premises and the like. They’re also often modelled with undersized doors. Fact is, many nominal 6mm buildings are really more like 4-5mm scale (and it doesn’t help that many nominal 6mm figures and models are really more like 7mm). So, I’ve adjusted my expectations and am now aiming for representing colony settlements and other more outlying patches of BUA where it might make more sense for buildings to be rather dinky (think light industrial type workshops, garages and the like).
As an aside, some of my 6mm sci-fi buildings I now intend to repurpose for 3mm sci-fi with some converting of doors and windows. Maybe someday I’ll do a proper city in that scale but it’s not a set goal for me.