Degenerate quantum fluids, such as the cosmic axion fluid, are commonly described by classical field equations. A formalism is developed to help calculate in quantum field theory the departures from the description of a system by classical field equations. The formalism is applied to a homogeneous condensate with attractive contact interactions and to a homogeneous self-gravitating condensate in critical expansion. In their classical descriptions, such condensates persist forever. It is shown that in their quantum description, parametric resonance causes quanta to jump in pairs out of the condensate into all modes with wavevector less than some critical value. We calculate the time scale over which the homogeneous condensate is depleted, and after which a classical description is invalid. It is argued that the duration of classicality of inhomogeneous condensates is shorter than that of homogeneous condensates.