Just did 2 nights with a few miles walk on the first day, and 10 miles on the second day. The mission was to check out some new kit. On the first day I necked a couple of ltrs of water prior to setting off
and then carried out just four ltrs. By the first night I'd consumed 1 ltr, and on the 2nd night I only had a few mouthfuls left, and the 10 miles was thirsty work.
There was a spring on the first night, and I often drink from them, but I'm always concerned with this location because of potential overflow contamination from heavy livestock farming further up the slopes.
I also ate lots of raw fruit and veg, but did eat some cooked food (which my digestion is not used to) which was salty. I was thirsty on the final morning and was glad for my 9am prearranged pickup.
I think therefore that I'd carry 3 ltrs per day when moving a fair bit in cool weather. In hindsight I guess I'd take out 7 to 8 ltrs for that trip, or maybe 5 ltrs if I had it sorted to make safe drinking water.