  We examine the time-dependent behavior of a nonlinear system driven by a
two-frequency forcing. By using a non-perturbative approach, we are able to
derive an asymptotic expression, valid in the long-time limit, for the time
average of the output variable which describes the response of the system. We
identify several universal features of the asymptotic response of the system,
which are independent of the details of the model. In particular, we determine
an asymptotic expression for the width of the resonance observed by keeping one
frequency fixed, and varying the other one. We show that this width is smaller
than the usually assumed Fourier width by a factor determined by the two
driving frequencies, and independent of the model system parameters. Additional
general features can also be identified depending on the specific symmetry
properties of the system. Our results find direct application in the study of
sub-Fourier signal processing with nonlinear systems.
