I take 5 minute showers in the morning before work and that's plenty of time if all you do is get clean. I don't find it relaxing to stand under the shower so all I do is...well actually shower

However. If nobody else needs to take a shower, if your daughter doesn't need to go anywhere, if there is no rush and the time is there...let her be. She clearly enjoys taking a shower for whatever reason and that is not by any means a bad thing. If she loves 20 minute showers plan it into her evening routine! I know I LOVED taking showers nd baths when I was a kid. It couldn't be long enough! Now as an adult I get in and out as quickly as I can
