Thanks dad.
AI get it at around 18 turns, give or take. In my recent test I disabled ruins and deleted any barbs coming my way.
1. Rush pottery (then animal husbandry and mining for potential extra hammers)
2. If you don't get monument you'll get your first social policy at turn 30 - so rush monument. In my recent test I got my first social policy at turn 16.
3. Between getting your monument and first social policy start making any building so once you do get your policy you can use the invest and overflow trick. Since you only have time for one building to do this trick on, if you're going to finish the building before getting your first social policy, try to work food tiles instead and time it that right before you'll get your first social policy you will also be one turn away from completing the building.
4. Get first social policy.
5. Finish the building from step 3 using the invest trick.
6. Start making stonehenge.
In this start I was looking to get it at turn 25, AI got it at 22 (which is actually pretty late from my experience from the AI).
https://imgur.com/a/aEE8v