It was in aCoS when Nicola approaches her for the blackmail incident. She asks Egwene to be given the same chance Egwene was, and Egwene counters with the arguments about forcing and how dangerous it was, and recalls Siuan apologizing for doing it to her.
In my opinion, that referred to her teaching session on the boat ride from Shienar, which is the only thing that matches the descriptions in context, but it could also be an indirect result of the situations into which she was flung and supposedly forced to learn faster, in other words, forced to force herself.
It can't be the trip from Shienar. We know Egwene was pretty weak once she came to the Tower. In her own words:
She did grow in strength when with the Seanchan.
Ahh, but when they were in Tarabon, and on the road, Elayne and Nynaeve had to channel cautiously because they couldn't risk being detected.
Egwene, on the other hand, could channel and hone her abilities without risk. In the Tower, Aes Sedai would have held her back, warned her of the risks and refused to let her push herself. But Siuan sent her away from all that, and that's why she feels guilt, I'm sure.
As of TSR, Egwene says she can hold enough of the Power that if Moiraine tried to hold as much, she'd be on the floor insensible. This indicates she's just stronger than Moiraine.
From hints Terez has dropped, Egwene had more or less reached her potential by the time she became Amyrlin. Moiraine in NS says it would take her and Siuan years to reach their full potential (from their current position of being just weaker than Anaiya). Egwene probably went through an even greater gap in strength in a few months, and that's because she was able to freely use the OP as much as she wanted, away from White Tower supervision.
