Time needed for incremental field differential refraction to exceed a given deviation, as a function of target declination. These curves are computed for an instrument similar to 4MOST, a φ = 3 deg FOV spectrograph at the latitude of Paranal. Southern fields can be observed for times longer than one hour without resulting in significant deviations. That is not the case for northern fields, where fibre reconfiguration will be needed within the first hour of integration.