Scalability is one of the most valuable features of the Desktop Grid concept. The “master-worker” computing model allows to gather huge computing resources with a single server. But still having in mind the quality of service, there are limits on the number of computing nodes that can be supported. In this paper, we consider a Desktop Grid as a queueing system. We construct a mathematical model and list the main quality-of-service characteristics. Using the log data of a volunteer computing project SiDock@home, we evaluate steady-state performance characteristics of the real-world project and estimate its peak performance and quality of service.