Flow monitoring has become an essential source of information for intrusion detection systems and various forms of network data analytics. However, the attention of researchers is focused primarily on the utilisation of the flow data, and the process of flow data creation is often neglected. This lack of consideration negatively affects the results of data analytics. Either the results are suboptimal due to the low quality of the flow data, or a description of the configuration of the flow monitoring system is missing, which leads to irreproducible results. The goal of this paper is to demonstrate how the configuration of the flow monitoring system affects the resulting data. The most basic flow monitoring configuration variables are the flow expiration timeouts. We analyse their effect on the number of created flow records to show their importance. Moreover, we demonstrate that the choice of the flow expiration timeouts can have a severe impact on the network data analytics. The use-case of Slowloris attack detection is used as an example to illustrate this fact.