Once a civilisation overshoots its biophysical limits, as ours did over 50 years ago

We did it back in the 1920s, when humanity exceeded 2 billion. That is our carrying capacity, assuming a near-zero impact on the planetary ecology (beyond which we enter into overshoot), and with a totally vegan diet.

It drops to about 500 million with a western-style diet involving meat. So technically it’s possible to pin it even earlier, at around the year 1650. But I like to be unreasonably optimistic sometimes, so the 1920s it is.