Several initiatives propose the use of opportunistic networks and heterogeneous devices to help overcome the communication and coordination limitations evidenced during first response activities in disaster relief scenarios. These solutions tend to create an Internet of Things ecosystem in which most components are mobile, autonomous and interact with other in a loosely-coupled fashion. Regardless the benefits provided by these infrastructures, the message delivery on them does not consider time constraints. This aspect is particularly relevant in this scenario since the time to conduct the first response activities is quite short, therefore they must be done quickly and coordinately. Trying to help address this challenge, this paper proposes a message propagation model for opportunistic networks that considers heterogeneous devices and guarantees the real-time behavior of the network by bounding the maximum delay for messages transmission. The message propagation is modeled using an analytical approach. Two different scheduling policies are used to analyze the model and their feasibility conditions are proved.