Search engine optimization is a monitoring & reading activity and totally depends upon the contents and structure of the website, so it is time consuming job. SEO is a continuous process, depends upon the update in algorithm of the search engines. Each search engine have their own algorithm technology. The search engine like Goggle loves unique contents with unique title tag, MSN gives higher appearance in search results for those having high link popularity and good title tag, Yahoo gives importance to description tags etc. etc., Therefore a site may rank high in one search engine and lower in another.
Nobody can say that search engines technology will always remain constant; it may change and improve as per the new search techniques and ideas. If an expert guarantees you about number one position for life time then that is a tip off that they are not really an expert.
Today’s market is very very competitive and if you want to command the market and want to become leader in the market; you should be always ahead of your competitors. In the internet marketing if you stop doing SEO activities then you may or may not remain in top 10 to 20 results. It is totally depends upon how much competition do your business have.
For eg. suppose my company is offering web hosting services and in today’s date web hosting field is very competitive. So for web hosting keyword today I am in top 20, but it is possible that I may not remain in top 50 results in future. But if you have less competition then your results will remain stable which helps you to increase the chances of appearing in top search engine ranking results.
Therefore there is no way to guarantee specific search engine rankings over a duration of time because constant "algorithmic" modifications within individual engines can cause placements to rise, slip or be lost all together.
As per my experience, SEO is a time consuming activity. In this field, results are not immediate, just conduct SEO activities and wait for few days to know results of your working. From last one year I am in search engine optimization field and I found that Google updates its database (google dance) every three month (may take more than three months also) and that’s why you can see changes in Google Page Rank for every three months. Search engine ranking positions of every search engine changes daily or weekly and its depends on how much competition the given keyword have.