This depends on the project, but in a traditional agency it depends per role within the company. For a first design stage typically the experienced designer needs approximately 10 - 16 hours for the first design, after that things tend to go faster for the additional versions. In addition account management teams work on the briefing, liaise with the client, and try to bring input. This increases the numbers to approximate double.
DesignBro’s take: We believe that the core hours should be focused as much as possible on the most important aspect: designing. That being said, the more experienced the designer is, the less there is a need for ‘advice’, ‘project management’ and other roles.