If you were really going for it, I'd say anything under 2:00 av. split is good for a fairly untrained girl, just for an idea a trained girl rower wil pull around 1:45, and less.
Training wise, don't bother too much with short pieces as the winter training has already started = looooong ergs, 18kms etc, so stick to 30 min pieces, 2 x 20min (1min rest), 3 x 20min (1 min rest), 40 mins with a rate change pyramid (every 5 mins), or straight up 60min ergos.