Photoinduced cis-trans isomerization has been used to measure the lifetime of structural heterogeneities in the matrices of poly(ethyl methacrylate) (PEMA) and poly(n-butyl methacrylate) (PnBMA) far below T-g. Nonequilibrium distribution of probe molecules over their environments was formed by a partial trans -> cis isomerization. To monitor the equilibration process, the back-isomerization was started after different delay times. The change in quantum yield of back-isomerization with delay time is interpreted to result from the change in the environment of probe molecules. The times required for the environments to change were estimated over a temperature range of T-g - 113 K to T-g - 73 K for PEMA and T-g - 65 K to T-g - 25 K for PnBMA. It was found that the lifetimes of different environments differ from each other by up to 2 orders of magnitude and correlate with the isomerization probability: the environment of the molecules which isomerize faster changes more often. For both matrices, the value of 55 kJ/mol was estimated for the activation energy of the lifetime of the least mobile environments while the lifetime of the most mobile environments depends on temperature only weakly.