The unofficial “61 day” rule from American Express is a widely discussed topic on credit forums and blogs.
A new American Express applicant can request a 3X limit increase 61 days from the day the account was opened.
Example: If you opened an account with AMEX on Jan 1 with a $5000 limit, on March 3rd (Jan 1 + 61 days), you can call in and request a credit limit of upto $15,000. Whether you get approved or not be determined by your credit worthiness, personal income and other credit factors. But if you are eligible, you will be approved for the $15,000 limit.