The isothermal volume relaxation rate in non-crystalline materials is analyzed for the Tool-Naraynaswamy-Moynihan phenomenological model, It is demonstrated that the mathematical analysis of the inflectional slope of isothermal relaxation curve yields in a simple equation relating parameters of non-exponentiality, non-linearity, effective activation energy and the stabilization period of isothermal volume relaxation response. It can be expected that in non-crystalline materials with comparable non-linearity the temperature dependence of the stabilization period is controlled by the parameter theta=Delta h*/RTg2. Materials with higher value of parameter a (organic polymers) exhibit relatively low relaxation rate. In contrast, higher relaxation rate is expected for materials with low value of B (inorganic glasses). The applicability of this approach for analysis of previously reported isothermal volume relaxation data of polyvinylacetate (PVA) and arsenic sulfide glass (As2S3) is discussed.