In France, to design drainage geocomposites into drainage and filtration systems, we have a new standard NF G38-061 (Afnor 2017). A major part of this standard is derived from a guide edited by the French Chapter of IGS (Comité Français des Géosynthétiques 2014). To design drainage geocomposites on the long term, the standard NF G38-061 (Afnor 2017) takes into account the decrease over time of geocomposite in-plane water flow capacity due to normal stress. Two phenomena are considered: the reduction of the composites thickness (compression of the drainage core) and simultaneously the intrusion of the geotextile filter into the drainage core in the case of soft boundary interface conditions. These two phenomena are assessed following two reduction factors that have to be applied to the in-plane water flow capacity of geocomposite measured following the standard NF EN ISO 12958 (Afnor 2010). There are current discussion in ISO TC 221 WG4 in the framework of the creation of the NF EN ISO 12958 part 2 - performance test, especially about the time of compression of the specimen in the cell before the in-plane water flow capacity measurement. This paper deals with the methods to assess the long term in-plane water flow capacity of geocomposites and discusses the time of compression of the specimen in the cell before the in-plane water flow capacity measurement is performed.