Not only KYC request but also different concern that consume Duelbits support time. Also most the casino doesnt have the instant verification feature for KYC unlike exchange offer because they manually verify the documents or submit it to the 3rd party who verify KYC for them. I experience undergoing KYC on other casino which took me a week to play again. I believe 3 to 7 days is the standard waiting of KYC procedure on Duelbits.
I don't think verification in centralized exchange is instant, it should be manually checked because it's easy for someone to create a fake KYC. Manual checking is used in order to see if the identity is true what he have submitted, if the face is different then of course the casino or exchange will reject it and ask to retake. Or they will ask more verification in order to more knowing the person behind the account.