A simple airplane-boarding model, introduced earlier by Frette and Hemmer, is considered. In this model, N passengers have reserved seats, but enter the airplane in arbitrary order. We are looking for an analytical expression, which describes the mean boarding time depending on the total number of passengers N. For this purpose, we first determine precise values of the exponents and expansion coefficients in the asymptotic expression at N →∞. It is reached by mathematical calculations and fitting the Monte Carlo simulation data for very large N, up to N ∼ 6 ⋅ 108. Finally, we compare the obtained analytical approximation to the simulation data for a realistic number of passengers N≲500 and find a good agreement.