It varies quite a bit. Some of mine have taken around 2 hours before I receive the registration email.
I would suggest just sitting tight and waiting for the email. After you receive this email the domain should appear in the registrations section of the panel.
Remember also; After the registration has gone through you will need to add the domain to the DreamHost hosting system via Domains -> Manage Domains -> Add New Domain / Sub-Domain. Assuming you intend to host the domain at DreamHost.
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