It tends to be under 50 for weight loss for most people, but plenty of people have been successful on Zone-style diets, too, which is typically 100-150g of carbs. The actual point for generating ketones is pretty much any point when you’re not eating enough carbs to meet daily glucose minimums. Dr. Michael Eades has said:
If you keep the carbs low enough so that the liver still has to make some sugar, then you will be in fat-burning mode while maintaining your muscle mass, the best of all worlds. How low is low enough? Well, when the ketosis process is humming along nicely and the brain and other tissues have converted to ketones for fuel, the requirement for glucose drops to about 120-130 gm per day. If you keep your carbs below that at, say, 60 grams per day, you’re liver will have to produce at least 60-70 grams of glucose to make up the deficit, so you will generate ketones that entire time.
He’s referring to fat-adapted people here, I think (“when the ketosis process is humming along nicely”); it sounds like if someone is not at that point the glucose requirement is even higher. But this all ties into the logic behind Atkins and Eades’ recommendations in their very low carb/keto plans that you can raise carb intake somewhat after a few weeks of extremely low levels and still continue to lose weight. Dr. Eades doesn’t make any reference to people needing to be extremely large to generate ketones at 60g/day. I would imagine that size makes some difference, because smaller people simply need less fuel. However, most of the glucose we make is for our brains and I don’t think our brain-size or power varies significantly with the size of the human.
Quote is from the last paragraph of this blog post:
https://proteinpower.com/drmike/2007/05/22/metabolism-and-ketosis/