We present a framework to approximate and improve the cycle time of semiconductor manufacturing fabs. We model the system as a generalized stochastic Petri-net and compare it to previous approaches, evaluate the advantages and consider this framework as an alternative to simulation models for practitioners. Using this framework the effect of applying certain control rules can be examined. These rules are often implicit and informal and are applied based on experience of operation managers with the goal of cycle time reduction. The present framework can provide the means to examine the effectiveness of such rules and help the operation managers make informed decisions regarding the application of such control rules.