Weekly water change for betta fish?

I just had one quick question about Betta fish actually. I am considering having an aquarium quite soon and have been to the local store to look at some fish. I have friends who have betta fish and they told me they changed their water weekly at least and they have had the fish for about two years now which I think shows that they do know how to care for the fish. However, when I went to the local store, I was told that a week was too often and that I should not change the water so often like that(changing the water weekly was often for the store owner). What do you suggest?