In my experience it took myself a week to start seeing whiter teeth and a month to see fully white teeth when brushing my teeth with baking soda.
To whiten your teeth with baking soda mix baking soda with enough fresh lemon juice (or water) to make a paste and then clean the excess saliva off your teeth with a paper towel before brushing your teeth with the baking soda paste.
Apply the baking soda paste to your teeth with a toothbrush and then leave it on your teeth for at least 1 minute then brush your teeth as normal and rinse your mouth out with water.
Brush your teeth daily with the baking soda paste and within a week you should see your teeth start going back to white and within a month you should see fully whiter teeth depending on how discolored your teeth are.