Laser-induced plasmas can be used to determine the multi-elemental composition of solid, liquid or gaseous samples. Using a pulsed laser beam focused on the sample, the resulting absorption leads to temperature increase of several 10,000 K, therefore to a neutral phase to plasma transition. During the relaxation phase over which the recombination takes place, a continuum then lines radiation is emitted. Its analysis based on Optical Emission Spectroscopy (OES) may lead to the composition of the plasma, therefore to that of the sample if fundamental assumptions are fulfilled [1]. The most restrictive one is the achievement of the Local Thermodynamic Equilibrium (LTE). Indeed, since only OES is used, the excited states number density is determined. Then, the ground states number density is obtained if LTE is observed and if electron temperature T_e is known, the electron density n_e being enough high. The characterization of LTE is therefore one of the key points of this analysis method, also known as Laser-Induced Breakdown Spectroscopy (LIBS) [2]. In this context, we have focused our attention on Argon-Hydrogen mixtures at atmospheric pressure. Using nanosecond laser sources (532 nm, 6 ns, 40 mJ), laser-induced plasmas have been produced with various Hydrogen mole fractions. The spectra have been recorded and calibrated according to the gate delay over different spectral regions. Large spectral windows have been used (around 60 nm) allowing a sufficiently high number of Argon lines. Using relevant radiation database, the spectra have been calculated assuming LTE by integration of the Radiative Transfer Equation and compared to the experimental ones for each spectral window. The temporal evolution of the electron density and temperature are then derived. The comparison of these evolutions from one spectral window to the other allows to estimate the uncertainty of the (n_e, T_e) determination and therefore to discuss the relevance of the LTE assumption.