In this Letter, the lifetime temporal reduction in ytterbium-doped aluminosilicate fibers due to pump-induced photodarkening (PD) has been reported. A linear correlation between lifetime reduction at equilibrium state and equilibrium losses due to the PD phenomenon was demonstrated. A squared-law dependence between lifetime reduction and dopant concentration (percent by weight) suggests a possible correction term for the rate equations system which allows improving its accuracy. Finally, a quenching effect was also observed for different pump power levels in order to find the corresponding lifetime value for different inversion levels. The quantitative analysis allows determination of the PD quenching term in the laser rate equations system to properly model a laser device that may be affected by this process.