How long does SEO takes?

Hi Good question SEO goes in cycles. Links are built, then they are crawled by SE's and indexed. But when the Search Engine crawls and indexes content it doesn't necessarily reoganize its index and find your new rank. With Google, the search engine will update its entire index about every four months. And since Google has well over half the market share we'll just focus on that one Search Engine for this post.
Let's suppose that we started a campaign in January, and Google just updated its index. You would be starting at square one. The exceptions being things like the age of the domain, age of existing links, strength and relevance of those existing links, and whether the links were anchor text or simply URL's.
If your hypothetical website called widgetworldusa dot com only had links pointing at it with the URL, it's link profile needs work. But if you widget site had anchor text linking to it for phrases you're pursuing, like "industrial strength widgets", then you're off to a good start.
Now about 3 months later you've built more "industrial strength widgets" links, but not all of them were crawled and included in the index yet. So on the first update you can hope for 25-50% of your links to have been picked up. These will go into consideration on the next update that's another 3 months away.
Now it's 6 months later and you're still hard at work on link building. This is the second Google update, with probably half or more of those links in the first update now included. But between update one and two you can again hope for 25-50% of your links to have been picked up.
Factors that generally play into how long does seo take can include the age of your domain, the relevance, the age of the link, the age of the link the domain is on, the age of the domain that the link is on, the authority of the domain your link is on, and much more.
Hopefully this provides a little insight about why SEO takes so long, and why many people offering SEO services usually require a commitment of 6 to 12 months to start a project.
For those wanting instantaneous results, Search Marketing and PPC is probably more suitable. But you should note that it accounts for less than 20% of the clickthroughs for search engine results pages (SERPs) and that the natural listings and SEO boast 80% of the clicks.
The best strategy is a mix of both, with Search Marketing to get you started until your SEO campaign catches up. It's also worth noting that your cost per customer and SEO ROI will decrease as time goes on. Because SEO provides a long term strategy for increased consumer presence, it has added value as compared to paid search marketing.

Ha ha ha.... Don't you laugh when you ask such questions...

Toooooo many factors has to be considered before we shoot our answer.