Due to the inevitable existence of quantum effects, a classical description generically breaks down after a finite quantum break-time $t_q$. We aim to find criteria for determining $t_q$. To this end, we construct a new prototype model that features numerous dynamically accessible quantum modes. Using explicit numerical time evolution, we establish how $t_q$ depends on the parameters of the system such as its particle number $N$. The presence of a classical instability leads to $t_q \sim \ln N$ or $t_q\sim \sqrt{N}$. In the stable case, we observe $t_q\sim N$, although full quantum breaking may not take place at all. We find that the different regimes merge smoothly with $t_q\sim N^\gamma$ ($0<\gamma<1$). As an outlook, we point out possibilities for transferring our results to black holes and expanding spacetimes.
