Another factor that gets missed with Google is site age, and visitor duration. The older your site is, the more trusted it is deemed so it will be seen as more important than newer sites. Also how long each visitor stays on your site is now much more important to Google. Try implementing some video content that explains that benefits of your sites that will keep the site visitors on longer.
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Getting that top position isn't hard if you are determined to work on it. However, keeping that position is much harder if you're using some popular keywords. Site age has some weight.
There are too many factors at play to give an accurate answer to that question. Competitve analysis comes into play. Ultimately you need one or all of the following, depending on the niche. 1. More pages on your site than the competitor at the number one spot for your keywords. 2. More high quality links than the competitor at the number one spot for your keywords. If you have both of these and you still are not ranking for your keywords, you need to take a second look at your own sites architecture, internal linking etc..
24hrs with correct SEO. Remember you cannot get top position for highly competitive keywords. Try targeting long tail keywords. I was able to get rank in top 10 for "free agent desk review" as soon as 36 hrs. I am still in top 10 in Google. Now I am in top ten in Yahoo and Live too.