In this study, two different natural fiber reinforced composites were characterized from the point of view of decomposition kinetics comparing two kinetic models, Kissinger and model-free kinetics, in order to estimate the lifetime of composites. Composite materials were prepared with low-density polyethylene as matrix and plasma-treated and untreated natural fibers were used as reinforcements, in two different amounts (20 and 30 wt%). Composites were manufactured using a rotor mixer and a hot plate press. Afterwards, a thermogravimetric analysis was carried out for each material at six different heating rates (5, 7, 10, 13, 15, and 20 degrees C/min) with a coupled mass spectrometry device to identify released elements in a specific temperature range. The influence of the low-pressure plasma treatment, as well as the fiber type, was taken into account to evaluate the activation energy of the decomposition processes of each material. Besides, lifetime was estimated from the obtained decomposition energies and the Toop equation. It was found that plasma treatment does not have a meaningful influence on decomposition kinetics, but the main composition of the natural fibers is decisive in this aspect, giving rise to much longer lifetimes when the cellulose content of the fiber is higher.