Let me get this straight, this isn't a fantasy, you had her breasts tied for two hours, then began to paddle them while still tied. Yes?

What was your goal for the session?
Were you trying to force her to safe word?

If you haven't already - please take a look at this link:
The Breast Punishment Primer
http://www.darkerpleasures.com/primer/

Depending on how tight she was tied, safewording much earlier might have been appropriate.

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Meanwhile, to answer your question, it's been so long that I can't remember the first time I used a safe word. Yet I do remember the first time my sub did. He said his word, "Mercy" and I was very proud of him for doing so.