  In recent years, a number of intensity-frontier experiments have been
proposed to search for feebly interacting particles with a mass in the GeV
range. We show analytically how the characteristic shape of the sensitivity
regions of such experiments - upper and lower boundaries of the probed region,
the maximal mass reach - depends on the parameters of the experiments, taking
the SHiP and the MATHUSLA experiments as an example. We find a good agreement
of our estimates with the results of the Monte Carlo simulations.
