A woman should shower as often as once per day if needed.
Although for some women it's fine for the woman to shower only once every few days or so unless you get dirty or smelly and need to shower more often.
A shower for a girl should take between 5 minutes to 15 minutes.
Some girls shower longer than 15 minutes but if you do then you risk drying out your skin.
15 minutes is really the max amount of time a girl or anyone should shower to avoid drying out their skin.
The age at which a mom should stop showering with her daughter is around 5 years of age.
It's recommended that a mother or parent stops showering or bathing with their child by school age which is 5 years old.
Some kids may need help bathing or showering to properly get cleaned up until around age 7 or 8 but they can take a bath or shower without the parents.
It's just that some kids fake taking showers and fake taking baths when they are 8 or under so a parent sometimes has to be in the bathroom with them to ensure they actually get cleaned up.
However some kids will take a bath or shower without faking it but it depends on the kid.
So you should make your decision about when to stop the joint showers based on your own level of comfort, along with some understanding of your child's development and level of comfort.
You will not cause any psychological harm by showering with her, unless you ignore her wishes when she tells you she wants to stop.
Parenting and child development experts do seem to mostly agree that parents bathing with children is normal and healthy up until the child begins to show discomfort or the parent themselves begin to feel concern.