depends on which registrar you choose and which TLD you want.
Some registrars can do it in near realtime for some TLDs, some may require manual checking of contact information and stuff, some TLDs require all kinds of paperwork to be sent in (signed forms, sometimes birth certificates or chamber of commerce registration forms, etc.) which can take days or weeks.
As people are clearly allowed to attack me but I'm not allowed to defend myself, I no longer post to this site.