It depends how hard the LEDs are being driven, optics, angle etc. The Knog pushes them pretty hard - hence the poor battery life on the higher settings (only 4-5 hours) - battery capacity is also not huge either. My powerful rear light (a torch) pumps out 150 lumens, but only lasts for 95-100min mins as it is being driven at 1.4A.
I worry more about being hit from behind (hence two powerful rear lights), but it's more likely an incident/collision will happen in front of you. For that reason, I'd go with a powerful front light too!