How long should the whitening effect last?

Because I like coffee, my tooth color isㅠㅠ

Whitening toothpaste doesn't have much effect, and it causes sensitivity and doesn't feel refreshing, so I want to get teeth whitening.

Is teeth whitening effective after just one treatment?

How many times do I need to do it for it to be effective?

I'm curious if it will cause any discomfort when doing it.