My impression is that the term is used somewhat loosely, and they normally mean "ran the account down to so little that there wasn't enough left to 'play on' meaningfully (or maybe 'not enough margin to open realistic positions') without re-funding the account."
If I understand what you're asking, here (no certainty!) I think the answer is "never".
Most of the people posting about this are trading spot forex against a counterparty whom they wrongly imagine to be a broker executing trades on their behalf in an underlying market.
Few will ever really "go negative".
Some well regulated spot forex "brokers" offer negative balance protection, anyway.
The concept of "counterparty" isn't really meaningful.
In the case of anyone using a genuine broker, their unknown "counterparty" isn't affected.
Some people who don't like demo accounts do their "experimenting" by using something like a $200 account at a broker like Oanda which offers infinite granularity of position-sizing, and treat it more or less as "play money", taking huge risks with it, to see if they can get it up to, say, $1,000, with the idea that if they do, they'll then start "taking it seriously" and determine more appropriate position-sizing. I
think.
Understandably enough, they can burn through several small accounts quite quickly, doing that.
(They can also totally fail to benefit in the sense that they might actually hit on something that has a genuine edge, but they'll never know that, and abandon it as "no good" because it blew their account. The reality, of course, is that "it" didn't blow their account:
they did.
)
They're not (normally) talking about $200
k accounts.
Maybe better just to ignore them completely, rather than worrying about exactly what they mean?