If they build it in their capital, the short answer is "much faster than we can build Stonehenge."
For a more specific answer, they have been making 16 food / 9 production in their capital for quite some time. This is a size 5 capital working irrigated corn (6/0), farmed bananas (4/0), grassland hill mine (1/3), plains hill mine (0/4), and a floodplains cottage (3/0), plus the default center tile on a plains hill (2/2). Added together and we get their 16/9 figure.
Now CFC built a settler on T34 and settled their second city on T35. Since they only had one worker at the time, they almost certainly did another worker in the capital next, which would have taken 4t given their 15 foodhammers. That takes us up to T38. If they started another settler after the worker, it would take 7t to complete, and they will be settling around T46, basically the same time we plant our third city. This is the most likely outcome, because CFC didn't lower their food output at all in recent turns, and their capital would have grown to size 6 and into unhappiness if they kept running that much food. They are probably building their third settler in the capital right now.
* If they are building a settler case: Capital finishes settler T45. They should be able to swap their banana tile over to a grassland or plains hill mine, for roughly 12 or 13 production/turn. This would have them completing Stonehenge right around our T55 date even without chops. With chops, they would win for sure. We will have a good indication if they are doing this because their Production will go up on the graphs. This has not happened yet.
* If they went for Stonehenge over third settler case: At their current 9 production/turn rate, they could have started on T38. That gives them 17 turns to build the wonder before we chop/whip it out - they finish on T52. I find this somewhat unlikely, but if they chose this path then we have no chance. Ditto for any other team that chooses to rush Stonehenge over planting a third city.
Keep in mind that CFC is Spiritual and can revolt into Slavery without the Anarchy turn. Basically, if they are serious about chopping and whipping out a wonder, they can win the race fairly easily. Our own T55 date is not especially fast. There's an excellent chance that some team will get there first because they didn't care about expanding and screwing themselves over long term. But we shall see.