This letter deals with microwave noise diodes in avalanche regime. Starting from the model proposed by Hines and Gliden in 1966, the asymptotic excess noise ratio (ENR) expression is derived for ω → 0. The developed theory is then validated against ENR measurements of three noise diodes already published in the literature. The agreement between theory and measurements is good and the obtained formula also predicts the 1/I ₀ behavior of the ENR at low frequencies. This study is relevant because it simplifies the experimental determination of the average time between two ionizations (the Hines τ _x model parameter) by means of low-frequency noise measurements only.