How long will white spots on tonsils last?

Depends. What you are describing are tonsil stones or tonsilloliths. These are usually associated with bad breath among other things (pain/difficulty swallowing, metallic taste, coughing, ear ache, swollen tonsils). The cause is not clear, but it is thought that food and bacteria tend to collect in little crypts on the tonsil leading to residual calcium deposits or stones. See your doc to be sure .