@hitender singh Thanks for posting in our Q&A.
To clarify this issue, we appreciate your help to clear something:
1.From your description, I know that the devices are in on-premises AD. So, what joined type did you want? Hybrid Azure AD joined(The devices are joined to on-premises AD and Azure AD)? Or Azure AD joined(The devices are just joined to Azure AD)?
2.If you want Hybrid Azure AD joined, autopilot enrollment and GOP enrollment could be used. However, autopilot needs to reset the devices. For GPO enrollment, we can read the following article as a reference:
https://learn.microsoft.com/en-us/windows/client-management/mdm/enroll-a-windows-10-device-automatically-using-group-policy
3.If you want just Azure AD joined, it is needed to remove the devices from the on-premises AD. Then enroll the devices to intune.
Not sure how long it will take, these work involve many factors such as the network, the device itself and so on.
If there is anyting update, feel free to let us know.
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