After you have registered your domain name and paid for it, it is reserved immediately. From then on, it is attributed to you and is not available for purchase by anyone else for the term of the registration.
The domain name then has to be technically available. This will happen within 24 to 72 hours later, assuming you have provided the correct technical parameters such as valid DNS servers.
At one time or another, you may have wondered, “why is my domain name taking so long to be registered?”. The answer is that not all domains are registered the same. Every domain is owned and operated by an entity called a Registry. For example, Verisign is one of the world’s most notable Registries because it operates two of the most popular domain endings .com domain and .net domain as well as others. Every domain is a unique brand and therefore has its own requirements and processes for registration so there are different time phrases for the question of how does long it takes to register a domain.