Do I have to change the brand of the shampoo I use after few months ?

I keep 2 different brands of shampoo at all times, when my hair starts to feel or look unclean after washing I switch over to the other brand, also if it looks dull. Another thing that I do every once in a while is to pour white vinegar diluted with water over my hair and then rinse out. This makes if feel squeaky clean.

No, you don't have to switch shampoos. Use an anti-residue shampoo or a clarifying shampoo once a week to remove hair products build up. Then use your favorite shampoo as usual.