The study of charge carrier decay dynamics in semiconductors plays an important role in determining various device properties, for example, the bulk minority carrier lifetime and the potential drop at interfaces. Time-resolved photoluminescence spectroscopy is an efficient technique to extract carrier decay time constants, with the simplest case being that of a mono-exponential decay controlled by a single type of recombination center. However, many modern materials often display multi-exponential decay behavior influenced by a broad distribution of defects. Utilizing CuIn1-xGaxSe2 as a reference system, we present a comprehensive and systematic theoretical study of the decay kinetics in a non-interacting two-defect-level system by using both the classical analytical approach developed by Hornbeck and Haynes and a homogeneous kinetic rate equation model solved numerically. We show how the trapping level's carrier capture cross-section symmetry factor and the initial injection level controls the parameter range for the agreement between these two approaches, analyzing the validity of multi-exponential fits for extracting the carrier lifetime. Further, we analytically and numerically demonstrate that information on the carrier recombination lifetime is contained in the fast time constant, whereas the agreement of the subsequent slow time constant with the analytical model depends on the energetic position of the defect level within the bandgap.