Most banks in the US will not accept a check 6 months after the date printed on it. However, sometimes it can be less if Google writes a different time frame on the check. Besides, why would you want to just hold onto the check without depositing it?
AdSense checks expire six months after they've been issued. If a check hasn't been cashed six months after the date that appears on the check, a stop payment will be placed on the check. When this happens, you'll see the line 'Check Expired - Earnings credited back' appear in your Payment History page. We'll then reissue these earnings in the next cycle in which you're eligible for payments. https://www.google.com/adsense/support/bin/answer.py?hl=en&answer=53067