I use the
Cateye Reflex Auto for my multi-day audax rides. According to the specs it only manages 30 hours on constant mode, but I feel like it actually lasts longer due to its ability to automatically turn on and off based on its light and motion sensors. I don't recall swapping batteries during my 1000k rides; I imagine even with 1200k or longer, carrying a single set of spares would be sufficient.
Though like many other lights of this kind, you need a way to secure both halves of the light together or they can come apart and fall off during a particularly bumpy stretch of road. I've used both rubber bands and metal wire twists successfully for this purpose.