As I said, I think the best timing for this is to add this to her "future plans" act 3 scene.
Personally, I'd be interested to see her reaction to saying it post grove/pre nightsong, but I think the best timing to say it and for her to say it back is the swimming scene, when she's casting off her past inhibitions and fears. I think she actually does fear those words, she fears committing to someone only to lose them or for them to leave her. It honestly brought me to tears a little to hear her say all those hopeful things when you talk to her in the final fight after a lifetime of hopelessness.
