Teeth Bleaching Question?

im getting my teach bleeched soon at teh dentist and ill only be [aying around $300 since the dentist is a good friend of my uncle's BUT my mum told me that it only works for three months and then itll go really yellow? i think shes lying? what really does happen and what type f food am i suppose to avoid? like be specific? drinks? im not suppose to drink tea right cos i drink a lot of tea and ketchup arent allowed too or anything with soy sauce, etc?

Suggestion:

After the bleaching process your teeth will remain at their new shade indefinitely. Certain foods can stain your teeth, but those are called extrinsic stains and can be easily removed. Your mother is incorrect in her assessment. Good luck