On fairly low current NiMH (Duracell 2100mAh), runtimes to 50% initial brightness on my L1S got 3hr 11min on Hi, and 8hr 21mins on low. Alkaline would be less than that, L91 lithium more.
Chevrofreak reported detailed runtimes here (do a search for the graphs), and according to my records, his was a little different from mine. On 2650mAh NiMH to 50% output, he only got 2hr 46min on Hi, but 12hr 26min on Lo. His L91 lithiums were 2hr 56 on Hi, 17hr 20min on low. His alkaline numbers were low, since he uses cheaper Kirkland alkalines (Duracells would run longer): they were 1hr 4 mins on Hi, 10hr 17 mins on low.