Go ahead & respond whenever you're ready. You don't have to wait for any specific amount of time. Just don't be predictable and ALWAYS wait or ALWAYS reply right away.
But totally take the lead of the interaction & change the subject at will. You don't have to stay with her subject of thought.
I'd tell her my day was awesome & give a quick upbeat explanation. (The great weather, seeing an old friend, having the best lunch ever, trying a new coffee that was amazing, etc....)
Anything just to share a moment of your day. Girls love that stuff. They like to feel as if they're a part of your world.