We present a simple tool to evaluate the dominant dynamical regime of a lava flow and to estimate the order of magnitude of the main rheological parameter (viscosity or yield strength) controlling the length of the lava flow with time. We consider three dynamical regimes: a Newtonian viscous regime, a yield strength-dominated regime and a crust-dominated regime. For each of these regimes, we present a scaling analysis to derive relationships between front position and time, emitted volume, slope, width of the flow and rheological properties. We apply the resulting equations to published data from eruptions of 10 lava flows with a range of compositions and conditions. Comparisons of the fits of the models to the data reveal that short-lived, high effusion rate eruptions are dominated by the internal viscosity of the lava, whereas low effusion rate or long-lived eruptions are dominated by the yield strength in the growing crust. Finally, blocky lavas with very high initial crystal contents are dominated by the internal yield strength. The evolution of some flows can be approximated with only two viscosity values: an early low lava viscosity stage and a later higher viscosity stage. The increase in viscosity is attributed to the initial disequilibrium conditions of the magma at the vent with further degassing and cooling triggering crystallisation of the lava flow. For yield strength-dominated flows yield strength is always within an order of magnitude of 10(5) Pa. This study provides a practical framework for predicting the evolution of the length of lava flows from estimates of the crystal content of the erupting lava and its effusion rate.