Cloud environments are being increasingly used for the deployment and execution of service-based business processes (SBPs). Among other properties, cloud provides elasticity in order to guarantee provisioning of necessary resources that ensure a smooth functioning of cloud services despite changes in solicitations. Provisioning of elastic infrastructures and platforms is not sufficient to provide elasticity of the deployed SBP. Therefore, SBPs should be provided with elasticity mechanisms to ensure their adaptation to the workload changes. In this paper, we propose a formal model, using timed Petri nets, for SBPs elasticity considering temporal constraints and an approach for time-based evaluation of SBPs elasticity strategies.