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In this paper, we investigate the applicability of different theories on the intensity-dependent ionization rate for C2+ atomic targets at different laser wavelengths (frequency) and at linear polarization. We use the analytical formulas and draw conclusions, from numerical comparison with the results from ab initio ‘two-state model’ R-matrix Floquet calculation, on their correct predictions of the ionization rate. The single-photon ionization has been studied in the vicinity of the 1s2 (2Po)2pns (1Po), n = 5–12 autoionizing resonances at non-perturbative laser intensity. The results obtained from Perelomov–Popov–Terent’ev and Ammosov–Delone–Krainov models are compared in a region away from resonance where the two-state model description is not as good. To quantify the deviation between theoretical models, we analyze the ratio between different data sets as functions of the Keldysh parameter. We conclude that the results obtained with the model of Perelemov–Popov–Terent’ev are the closest to the ab initio R-matrix Floquet calculation.