I am looking into treatments for melasma and blemishes. How long does it last after just one session?

Lately, my face looks so patchy all over, and I really hate how it looks. I've noticed a lot of my neighbors going to dermatologists to get their melasma and blemishes removed, so I'm actively researching whether I should get the procedure done as well. However, I've heard that one session isn't enough and you have to sign up for a multi-session package. I don't have the time to go often, and it's a hassle, so I'm wondering if getting just one proper treatment for melasma and blemishes would help maintain clear skin for a while? My biggest worry is that after spending a lot of money, the melasma and blemishes will darken and reappear within a few months. I would appreciate some advice from those who have experience with this.