Google adsense took a long time to review your website in order to allow google adsense why?

If you encountered problem having your website to have google adsense ads to display, chances are your site is in violation or against the adsense TOS policy.
Most of the time it have to do with your site had scrapped information data from another website, yes google can detect that. If you have contents on one site and now you’re bring it into another site, and so you’re ending with two sites having the same contents google will know that. So does both site get flagged? not really, I think it’s the second one third one and so on get flagged while the first one get its original contents.

There are websites out there that will help you determine where your duplicate contents coming from on which websites and so on, although not exact copy but if certain # count of words like 50 or more similar words (very smart algo. to detect) then your site is flagged. Use site like Copyscape to check your website contents before asking google to check to see if your website is ready for google adsense.

In the past, and maybe still true today, once you have google adsense account, you can place the google code ads to any website, anywhere, but could resulting from a no paying message or warning or even suspension of your account because you violated google adsence ads placement policy on certain websites and its contents. Potentially on bad websites you could also ding for when the advertisers reviewed your website and complained that your website is not legit you could get ads paying deducted, meaning earning get reversed.

So it is better to have google screen your site first, and get a confirmation message before you place ads on that site.

The team has now reviewed it but unfortunately your site isn’t ready to show ads at this time. There are some issues which need fixing before your site is ready to show ads.
For more information, visit the ‘Sites’ page in your account.
You can find more details and application tips in this help article.
We encourage you to review and update your site. Once you’ve done so, you can request a review. Our specialists will review your site for compliance with our Program policies, so make sure to resolve all of the issues.
For more information, please visit the AdSense Program policies and AdSense Terms and Conditions.

Your site isn’t ready to show ads
There are issues which need fixing before your site is ready to show ads.
Your site isn’t ready to show ads
We’ve found some policy violations on your site which means your site isn’t ready to show ads yet.

Valuable inventory: Scraped content
As stated in our program policies, we may not show Google ads on pages with little or no value and/or excessive advertising to the user. This includes pages that are scraping or rewriting of content from other sources without adding value. Please review Google’s Webmaster Quality Guidelines (http://www.google.com/support/webmasters/bin/answer.py?answer=66361) for more information.

For more information, please review the following resources:

Policy tips for creating high quality sites (part 1)
Policy tips for creating high quality sites (part 2)
Webmaster quality guidelines
AdSense program policies

View the Content policies or visit the Help Center for more information. After you’ve fixed the violation, you can request a review of your site.