The answer to this question varies on many factors. A recently created website would take way more time to rank than an established brand that already has some organic exposure. There are also industries that are more competitive than others. A website that hasn’t done any SEO before, might need 1-2 months of technical and on-page optimization, and then move to more rigorous SEO activites.
Why is this question important? It gives you a good understanding of how experienced your potential SEO consultant is? If you get an immediate answer as it will take 1 month or even up to 3, you will understand that this person hasn’t done any research on your industry or analyzed your website in detail.