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.