It seems that they say "desu/masu" is polite form and "da" is casual form. I believe, however, as a native speaker, desu/masu is usual (or normal, or normally casual) form, and "da" or so-called 'タメ口' is an unusual form which is used only among close friends where anything goes. So I believe you don't have to switch it. You can have intimacy even using 'desu/masu' form because it is usual. If you get into close relationship/friendship with others, sure you will know when to switch your speaking form on your own without knowing rules, because it is close.

It is difficult to decide when to switch to casual speaking even for Japanese. There are no rule . Of course nobody tell you. We just read between the lines. So,I think it depends on people’s personality . In my case , I hardly use casual form when I talk with the people who are older than me except close family . And I don’t use casual form at work. I just use it when I talk with friends who are almost same age or younger than me.