Google will likely keep your information until the end of time - or until they see fit.
Google has hundreds of millions of users (350 as of April 2012 - Larry Page), a number which has likely grown substantially. When you delete your account, Google will likely move this information off of their live servers (eventually, some mention that this is after 60 days). However, it will then remain on their backup servers, and their backup server backup servers and so forth. It wouldn't make sense that an automated system is in place to synchronously remove your information between the live servers, backup servers, backup server backup servers etc. And this certainly isn't performed by a human being considering the magnitude of registered users.
When you signed up for your Google (or back then - Gmail account) you agreed to the Google Terms of Service. Here is the archived version from April 16 2007.
In the archived version, you'll notice under section 19 - Changes to the Terms:
19.2 You understand and agree that if you use the Services after the date on which the Universal Terms or Additional Terms have changed, Google will treat your use as acceptance of the updated Universal Terms or Additional Terms.
In your post you mention in the present tense that you are thinking about deleting your account. This means that your continued use of their service(s) indicates your acceptance of the universal terms / additional terms.
As such, if you look at their current Terms of Service, under
Your Content in our Services:
When you upload, submit, store, send or receive content to or through our Services, you give Google (and those we work with) a worldwide license to use, host, store, reproduce, modify, create derivative works (such as those resulting from translations, adaptations or other changes we make so that your content works better with our Services), communicate, publish, publicly perform, publicly display and distribute such content.
It goes on further to say:
This license continues even if you stop using our Services (for example, for a business listing you have added to Google Maps).
Which means that even if you terminate your account or stop using the service, the license that you provided Google with continues with full legal standing.
Long story short, Google is not legally expected to terminate your information from any of their servers, and this information can be used by them at any time without notice for the purpose of their own development, or at the request of legal bodies.
Scary, isn't it?