Lets say I want SEO to be done on my app. The app is for finding local Plastic surgeons.

I want to know how the SEO guys work. Do they guarantee the results? How do they charge as in do they guarantee results for a time frame, say, that my app will come up in the top for atleast 6 months or for an year.

What would be the charges if I want the results on asap basis?

Thanks!