we are not able to tell you exactly how long it takes. But from what we experienced, it will be approved faster if you pay using Google Checkout and PayPal.
Some people get their accounts activated within one day.
Howerver, it really depends. If DH detects credit card fraud from somebody in your region, DH may take more time to active your account.
If you paid using Google Checkout or PayPal and your account is still pending for too long, you can always log into DH panel and contact support via support --> contact support
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