Data grid provides scalable infrastructure for storage resource and data files management, which supports several scientific applications. Replication is a technique used in data grid to improve the applications'response time and to reduce the bandwidth consumption. An important problem to be addressed is when replication should be trigged. In this paper, we propose a model for a dynamic period that is used to trigger replication by adapting to the grid behavior. We also evaluate the performance of the proposed period using simulations. Our results show that the dynamic period is more effective than the static period.