In the modern era, my production centers are normally bringing in 60 or so base hammers per turn. That usually requires flipping into State Property, although in BTS you may be able to get close to that with a Levee and lots of river. Roughly speaking, 3 hammers per pop point.
In the Ancient Era, you may be able to reach that level, but with a much lower population. For example, if you have irrigated corn (6F), then you can feed 6 grassland mines, which would get you to 19 base hammers per turn. The hard part, of course, is finding the terrain you need to pull this off.
If you are planning a primarily peaceful approach through the early eras, I'd say that you should be able to meet your military needs with a single city that brings in 15-20 base hammers.
For non production cities, I figure you need to have 8 hammers per turn available when you are building infrastructure, but probably not much more than that.