  The widely-used locally constant field approximation (LCFA) can be utilized
in order to derive a simple closed-form expression for the total number of
particles produced in the presence of a strong electromagnetic field of a
general spatio-temporal configuration. A usual justification for this
approximate approach is the requirement that the external field vary slowly in
space and time. In this investigation, we examine the validity of the LCFA by
comparing its predictions to the results obtained by means of exact
nonperturbative numerical techniques. To benchmark the LCFA in the regime of
small field amplitudes and low frequencies, we employ a semiclassical approach.
As a reference, we consider a standing electromagnetic wave oscillating both in
time and space as well as two spatially uniform field configurations: Sauter
pulse and oscillating electric field. Performing a thorough numerical analysis,
we identify the domain of the field parameters where the approximation is well
justified. In particular, it is demonstrated that the Keldysh parameter is not
a relevant quantity governing the accuracy of the LCFA.
