"She eats in a second."
Translation:היא אוכלת בשניה.
It's correct. I'd just add that "תוך שנייה" is used more and sounds better in my opinion.
Tip to help people remember this: You know how the word "second" can refer to both the period of time and something being number two in English? This word is also derived from the same root as the word for שניים, and the -i at the end can be thought of as the same "of or pertaining to" ending found in words like צמחוני and חקלאי ("plant" > "vegetarian", "field" > "farmer", "two" > "second")
So you have שני, meaning "second" in the masculine, הספר השני ("the second book"), and then the feminine form of that word is used to refer to the unit of time. So השנה השניה is a completely valid thing to say ("the second year" as in the one after the first).