For fixed c, the Prolate Spheroidal Wave Functions (PSWFs) ψ n,c form a basis with remarkable properties for the space of band-limited functions with bandwidth c. They have been largely studied and used after the seminal work of D. Slepian, H. Landau and H. Pollack. Many of the PSWFs applications rely heavily of the behavior and the decay rate of the eigenvalues (λ n (c)) n≥0 of the time and frequency limiting operator, which we denote by Q c. Hence, the issue of the accurate estimation of the spectrum of this operator has attracted a considerable interest, both in numerical and theoretical studies. In this work, we give an explicit integral approximation formula for these eigenvalues. This approximation holds true starting from the plunge region where the spectrum of Q c starts to have a fast decay. As a consequence of our explicit approximation formula, we give a precise description of the super-exponential decay rate of the λ n (c). Also, we mention that the described approximation scheme provides us with fairly accurate approximations of the λ n (c) with low computational load, even for very large values of the parameters c and n. Finally, we provide the reader with some numerical examples that illustrate the different results of this work. 2010 Mathematics Subject Classification. Primary 42C10, 65L70. Secondary 41A60, 65L15. Key words and phrases. Prolate spheroidal wave functions, eigenvalues and eigenfunctions approximations , asymptotic estimates.