If we talk about white-hat SEO process specifically, it can take anywhere between 6 months to 1 year to get visible SEO results after optimizing your website. In rare cases, it could happen in 3 months if the SEO expert is optimizing your website for keywords with low competition. If anyone is promising you SEO results in any less time, please understand that he/she is going to use black-hat SEO techniques which is only bad for your website. While white-hat SEO bring long term results, black-hat SEO can negatively affect your website which could be irreversible.