Offer available to customers with a billing address in the United States, Canada, United Kingdom, Australia, New Zealand, Thailand, Malaysia, Indonesia, Singapore, Denmark, Sweden, Switzerland, Norway, Portugal, Austria, Germany, Spain, Finland, Greece, Ireland, Italy, South Africa and the Philippines only. One promotional code per advertiser. Promotional code will automatically be applied to the new account within 24 hours from account creation.
To earn the credit: your advertising campaigns must accrue costs of a minimum spend, excluding any taxes, within 30 days. Making a payment of the minimum spend is not sufficient. The tracking of advertising costs towards the minimum spend begins once the code has been applied.
Once 2 & 3 are completed, the credit will typically be applied within 5 days to the Billing Summary of your account. Credits apply to future advertising costs only. Credits cannot be applied to costs accrued before the code was entered. You won’t receive a notification once your credit is used up and any additional advertising costs will be charged to your form of payment. If you don’t want to continue advertising, you can pause or delete your campaigns at any time.
Your account must be successfully billed by AdWords and remain in good standing in order to qualify for the promotional credit.

Full terms and conditions can be found here: https://www.google.com/adwords/coupons/terms.html