A new method is proposed for measuring the lifetime of heterogeneities in a glassy matrix. UV-vis absorption spectroscopy has been used to monitor the kinetics of photoinduced cis -> trans isomerization of 1-naphthyl-azomethoxybenzene (NAMB) in o-terphenyl (OTP) below T-g The dependence of isomerization rate on the duration of dark interval after generation of cis molecules was established; an increase in the dark interval causes a decrease in isomerization rate. This dependence is shown to be due to the change in the local environment of NAMB molecules. The time required for the environment to change was estimated over a temperature range of 234 (T-g - 9 K) to 241.5 K (T-g -1.5 K). The change in the environment of the guest molecules has been interpreted in terms of the exchange processes. The values obtained for the exchange time coincide with the rotation times of the NAMB molecule. Also, it is shown that the time of structural rearrangement of the environment near the fast reacting molecules is of the same order of magnitude as the time of structural rearrangement of the environment near the slow reacting ones.