Named Data Networking (NDN) has emerged as a promising candidate for shifting Internet communication model from host-centric to content-centric. A core component of NDN is its stateful forwarding plane: Content Routers keep track of pending requests (Interests) storing them in dedicated tables at routers (Pending Interest Tables). A thorough analysis of PIT scalability is fundamental for deploying NDN as a whole and questions naturally arise about memory requirements and feasibility at wire-speed. While previous works focus on data structures design under the threat of PIT state explosion, we develop for the first time an analytical model of PIT dynamics as a function of relevant system parameters. We provide a closed form characterization of average and maximum PIT size value at steady state. We build an experimental platform with high speed content router implementation to investigate PIT dynamics and to confirm the accuracy of our analytical findings. Finally, we provide guidelines on optimal PIT dimensioning and analyze the case of an ISP aggregation network with a trace-driven packet delay distribution. We conclude that, even in absence of caching and under optimal network bandwidth usage, PIT size results to be small in typical network settings.