The Inside AdSense blog says Time limit for AdSense referrals earnings is now 180 days. In that blog entry they source a thread from WebmasterWorld, experiencing disgust the the 90 day short window. We covered this back in late January and no one was happy with the 90 day window.
So AdSense has increase that window to 180 days, which is much more reasonable to earn a commission on an AdSense referral that earns $100 or more.
Forum discussion at WebmasterWorld.