This paper is devoted to model Safety Instrumented Systems (SISs) availability by Dynamic Bayesian Networks (DBNs). The models integrate several parameters but the main concerns of the study are the integration of test duration and test strategy. The proposed DBN are generic and can be reused for assessing performance and testing the effect of some parameters. More attention has been paid to the performance of the proof test, its harmlessness and particularly its duration. The duration increases the model complexity when considering the components availability given the test but it is more realistic. This parameter should be decided carefully to satisfy the Safety Integrity Level (SIL) objectives.