ggmoyang
Emperor
In some video we saw it takes 15 turns to outpost grow into a city.
These were done while they're healthy, which gives +20% to growth.
So I assumed it would take 18 turns while unhealthy. (There's no proof of this.)
In Solargamer's recent African Union video, it took 12 turns with Homesteading virtue(+30%) and being healthy. 18/(1+0.3+0.2)=12, sounds right.
So I made a reference here. +150% is achieved with 2 bonus already mentioned plus KP sponsor(+50%) and connecting trade route(+50%)
If you're not KP and don't have any bonus, connecting trade route will shave precious 6 turns. But if you're KP and either healthy or have homesteading, connecting trade route will only save 2 turns and probably not worth it.
Edit: If outpost starts with additional tiles(i. e. you settle them next to your border), it becomes city faster. It maybe better to count turns needed per a tile(base 3.6) instead
These were done while they're healthy, which gives +20% to growth.
So I assumed it would take 18 turns while unhealthy. (There's no proof of this.)
In Solargamer's recent African Union video, it took 12 turns with Homesteading virtue(+30%) and being healthy. 18/(1+0.3+0.2)=12, sounds right.
So I made a reference here. +150% is achieved with 2 bonus already mentioned plus KP sponsor(+50%) and connecting trade route(+50%)

If you're not KP and don't have any bonus, connecting trade route will shave precious 6 turns. But if you're KP and either healthy or have homesteading, connecting trade route will only save 2 turns and probably not worth it.
Edit: If outpost starts with additional tiles(i. e. you settle them next to your border), it becomes city faster. It maybe better to count turns needed per a tile(base 3.6) instead