@Rik - 6 turns iirc.
@Matrix: These cities cut down unit support and can produce workers/warriors as well as allow us to have an ample number of taxmen/scientists apart from generating 1 gold(science) per turn. They are utterly corrupt but thanks to our agricultural trait and presence of fresh water on the Iron Island, this will allow for a few extra population. There is little point in allowing them to grow to decent size because building improvements there would take too long and will be a waste of resources. But these cities can grow to pop 4-5 with 2-3 tiles worked thus allowing to hire specialists.
Well, if the enemy wants to go after these cities - good, we won't regret too much a loss of a few of them except that we have to protect the iron mountain by all costs.
For example, a city with an irrigated cow on plains in its radius and an irrigated grassland tile will have 3ftp (city tile) + 4fpt (cow) + 3fpt(grassland) for total 10 fpt and can support pop 5. Out of these 5, 2 can be taxmen or scientists. Assuming, we have slider set to 80% tax, this gives 1 gpt (city tile) + 1gpt (unit support) + 4 gpt (taxmen) total needing no military police or 1 mp for +5-6gpt. If we have 10 cities like this, this amounts to 50-60gpt. In case if slider is set to 80% science, it will generate up to 7 beakers/turn. We also need to claim all hills/mountains/desert tile for they might contain saltpeter.