SEO is an ongoing process as your competitors always try hard to fetch your place on the ranking table, hence executing SEO for a particular time period is never a promising idea. On the other way round, one cannot expect that he/she will invest once & rule the 1st place in search engines for life-time. Nothing is permanent, neither the rankings, nor the algorithms. Google keeps on updating its algorithm every now & then for better user experience; hence one needs to keep his SEO strategy implementation at its best. In short aprox 2 Months to 6 Months
Although we have projects where we have delivered good results within 2 months, but realistically we can expect around 30-40% of keywords on the 1st page of search engines. Organic SEO takes time as we will be earning ranks & not buying them. Any legitimate company will quote around the same time. Anything earlier than this will be a bonus.
We can guarantee our efforts, but not results. In the world of SEO, guarantee is a speculative term and many SEO companies have been using this to get more business, but the bitter truth is neither them nor us have the control over search engine algorithms as it is a third party platform and hence offering guarantee will be nothing more than a dummy commitment. Do not fall into this trap if anyone has offered you a ranking guarantee in search engine.