Injury layoff of a week or so--ho long to regain form?

I have a friend who has a shoulder injury (mild but enough that she doesn't want to risk delaying recovery to swim). Have any of you had layoffs from swimming due to injury or other issues and when you came back to the pool did it take you long to regain your form. She is freaked a bit because she loves swimming and is so afraid she will lose all the fitness she has gained over the last few months. She has done short swims of 1750 or so, maybe once a week. So not a significant layoff. But maybe a week or so. Any help or positive encouragement is welcome! thank you!