For 24-hour and adventure races with a basecamp, I run an OutBound Trail lamp on the bars, and run either a 2-cell or 4-cell pack. 2-cell pack nets about 4 hours of use on low, and 4-cell pack around 8-10 hours on low. The low setting has been sufficient for the majority of use, with only briefly stepping up to medium or medium-high for technical and/or high-speed descents.