The procedure of reliable extraction of average decay lifetimes was demonstrated with an example of slow non-stretched exponential photoluminescence kinetics from multilayers of Si nanocrystals and three different fitting models, namely: 1) multi-exponential, 2) combination of mono- and stretched exponential as well as 3) lognormal distribution of decay rates. All three decay models gave the same average decay lifetime 68 +- 1 µs.