Top: relation between flare duration and relative amplitude for all validated short-cadence flares on stars with measured rotation periods. The solid black line represents the duration–amplitude relation from Hawley et al. (2014), which we extracted from their Fig. 10. The dashed lines give the lower and upper envelope of their relation. Bottom: relation between flare duration and absolute peak flare amplitude. Our linear relation (Eq. (4)) is given as the black solid line.