Computer simulation is a widely used tool for analyzing many industrial and service systems. However, a major disadvantage of simulation is that the results are only estimates of the performance measures of interest, hence they need careful statistical analyses. Simulation studies are often classified as either terminating or non-terminating. One of the major problems in non-terminating simulations is the problem of initial transient. Many techniques have been proposed in the literature to deal with this problem. There are currently a number of studies to improve the efficiency and effectiveness of these techniques. However, no research has been reported yet that analyzes the behavior of the transient period. In this paper, we investigate the factors affecting the length of the transient period for non-terminating simulations, particularly for serial production lines and job-shop production systems. Factors such as the variability of processing times, system size, existence of bottleneck, reliability of system, system load level, and buffer capacity are investigated. Recommendations for the use of a new technique are given. A comprehensive bibliography is also provided.