In order to become eligible to receive payments for your AdSense account, all publishers are required to enter a Personal Identification Number (PIN). PINs are printed on white 4.5 x 6 inch (11.4 x 15.2 cm) postcards and sent via standard mail. The picture below is a preview of what the PIN mailer looks like:
Why do I need a PIN?
PINs are necessary to verify the accuracy of your contact information and to help ensure that you can receive your payments.
When should I expect to receive a PIN?
Google will print and send your PIN via standard mail several days after your account balance reaches the verification threshold (€10 or $10) for the first time. PINs typically arrive within 2-4 weeks, but may take longer depending on your location.
How long do I have to enter a PIN?
You'll have 6 months from the initial issue date to enter your PIN. If you have not entered a PIN within 4 months, Google will replace the paid advertisements on your site with unpaid Public Service Ads. If you still have not entered your PIN within 6 months, your account will be disabled and your unpaid earnings will be refunded to the appropriate advertisers.
What if I do not receive my PIN?
If you do not receive your first PIN, you can request 2 replacement PINs. If you have requested your maximum of 3 PINs and have not received any of them within 90 days, then sign in to your account. You'll see a notice on the Reports Overview page with further instructions.
In my future posts, I will share with you " How you can request for a new PIN ", "How you can remove hold from your Adsense account without a PIN" amongst other Adsense tips. Just make sure you SUBSCRIBE to this blog, if you do not want to miss them.
Source; Adsense Help