With proper care shower curtains can last up to a year and beyond! Since shower curtains aren’t terribly expensive replacing them is an easy way to breath new life into your bathroom. If you notice tough mold or stains they should be replaced immediately, but barring that replace at your own whim!
To avoid having your bathroom become a mildew mess consider what the causes of mildew are. It comes mostly from damp and wet stuff such as your tub and shower curtain. Bacteria builds up and spreads once the mildew is allowed to grow. This can cause illness and can seriously effect some people in the worst ways especially if you have allergies. To prevent mildew and to promote a clean and safe bathroom I suggest you:
1. Keep your bathroom well lit. Try to let in as much natural light as possible and if you can't then use electric light but try to shine it as much as possible all over the whole bathroom, even in the corners.
2. After using the shower or even if you just took a bath, open up the window and get some fresh air into the room. Let the moisture out.
3. Don't let your shower curtain stay opened after a shower. Close it back up again so that it straightens out and drips dry quicker.
4. Clean your bathroom often. Maybe once a week should do the trick. Make sure to clean your shower curtain with hot water and a little bleach.
5. Repaint the walls if mildew build up is out of control. Use the mildew proof paint and it should work like a charm.