Whitening is not a permanent result. How long it lasts depends above all on what happens afterwards, and the first few days are the most sensitive.
The first few days
It is worth reducing strongly staining food and drink: coffee, tea, red wine, dark sauces. Avoid tobacco.
If sensitivity appears, it is usually temporary and settles over the following days. If it persists, mention it at the clinic.
In the medium term
Daily hygiene and regular cleanings remove surface stains before they settle. They do not restore the whitening shade, but they slow its loss.
A realistic expectation
The rate at which the shade fades varies a lot between people. It is more useful to think of whitening as something maintained rather than something done once.