how often do u change your shampoo and conditioner?

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I have always been using Pantene since i really love the effect on it to my hair..It makes my hair so soft and bouncy which is what i really want it to be. But ive noticed that after several months from using it, im starting to have danruff and my friends told me i should change it every now and then. I am worried if i will switch to another shampoo it might damage my hair? I want to make sure that its really good for my hair before trying anything new..any suggestions?