Builder is a status given to an established and trusted player. From the point of view of the player, there is no difference between having full Builder status and Probation status. Builder status is given after proving oneself to be trustworthy. There is no fixed period of time to this, it normally just happens when one of the Admins remembers.
If a builder donates to the server, he is given extra commands and can choose his own in-game title.