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From the Google AdSense blog:
Time limit for AdSense referrals earnings is now 180 days... Based on the feedback we heard [webmasterworld.com], we agreed that 90 days may not provide enough time for your referred publishers to complete earning $100. Therefore, we decided to double the window. This change is retroactive...
[edited by: martinibuster at 8:24 pm (utc) on Mar. 2, 2006]
Also, I agree that 180 days still isn't enough. Adsense is very hard to make money with if you're just getting started and don't have a high-traffic site. While it is a step in the right direction, it is still a move by Google to make money off of many smaller sites without ever having to pay a referral fee to the publisher. I second wyweb.