In environmental investigations the effects of relatively long exposure times, often over a lifetime or decades, are of particular importance. Considering the importance of exposure time for toxicity to occur, the relationship between the lethal exposure time (LT50) and lethal exposure concentration (LC50) has been evaluated over relatively long exposure times using Normal Life Expectancy (NLE) as a reference point. The innovatory approach of using NLE as a reference point is important since it limits the data to the range where toxicity occurs. A model based on this concept has been developed which has the normal life expectancy (NLE) as a fixed limiting point for a species. The model is based on the equation (LC50 = a ln(LT50) + b) where a and b are constants. It was evaluated by plotting lnLT50 against LC50 with data on organic biocides with fish and metal, metalloid and organic compounds with zooplanktons obtained from the scientific literature. Most of the experimental data sets can be satisfactorily correlated by use of the RLE (Reduced Life Expectancy) model, but deviations occurred for some data sets. Those data sets were satisfactorily fitted by a Two Stage RLE model. This model was based on two phases: one in the peripheral system and other in the central system. Both the Single and Two Stage RLE model support the hypothesis that toxicity is time dependent and decreases in a systematic way with increasing exposure time. A Calculated NLE was derived from the plots. The Calculated NLE obtained was in good agreement with the Reported NLE obtained from literature. Estimation of toxicity at any, particularly long exposure time and concentration is possible using the model.