This is the acid test, because experienced SEO companies will tell you there’s no single right answer. Others may make promises they either can’t keep or will use questionable tactics to keep.
SEO work is done in phases. The first phase focuses on technical SEO and on-site optimization, and correcting any issues that are found. Some results from this initial phase may be seen in three or four months. Subsequent, ongoing work focuses on building fresh, relevant content and link development, which will have results that build over a long period of time. In any case, much depends on the state your website is in at the time SEO work begins, so everyone’s mileage may vary.