Removing stains from teeth can be achieved with the use of baking soda. The frequency with which this can be done depends on your sensitivity, so it will take a bit of testing. I started out doing this twice a day and after three days my teeth started hurting when I ate certain foods. So I cut down to three times per week and there were no adverse effects.
The stains on the front of my bottom teeth have disappeared and other stains in between my teeth have gone away as well. What I do is take baking soda and pour a half spoon of it into a small dish. Then I wet my toothbrush and dip it into the dish and then spread the baking soda on the bottom teeth. After spreading, I use my finger to “rub” it into my teeth and continue this rubbing for about 30 seconds. I then repeat the process with my upper teeth. Try not to brush aggressively with your toothbrush, just use it to spread the baking soda. As you experiment with frequency you will find how much you can tolerate, everyone’s different with regard to this!