Whitening toothpaste typically doesn't work instantly. Experts say that if you brush with whitening toothpaste twice a day it can take anywhere from two to six weeks for teeth to get whiter. There are some whitening toothpastes, namely those that contain an ingredient called covarine, that can work quicker. However, you'll want to follow the manufacturer's instructions carefully because some harsher whitening toothpastes can weaken enamel.