I can share something from my own experience that might help answer your question. My twin actually felt it from the very beginning, even though he couldn’t fully name what it was. He kept talking about how the universe orchestrated our meeting, how there was something pulling us together, his words, not mine. This was happening way before either of us knew what twin flames even were.
Knowing something is special and accepting what it means are two different things though. For three years, he acknowledged this heart pull while simultaneously fighting against what it represented. He’d verbalize the connection constantly (without using the exact words) but then retreat behind every excuse and label that kept him ‘safe.’
So to answer your question: I think the runner often knows on some level from the start. They feel it’s different, they sense they can’t connect with others the same way, they might even say it out loud like yours did.
But the full realization is probably scary to them. If they’re ready to accept the spiritual nature of the connection, then you’re probably very close to reunion by that point.