I started out with a filled Dromedary bag. But I also filtered lake water for use daily. I don't remember how long the civilized water lasted, but it wasn't that long. In general you will use 2 to 3 liters of water a day depending on how hot it is.
The Lake water is probably clean enough to drink unfiltered but I wouldn't trust it that far. I filtered water from the middle of the lake during the day when I was away from shore. I just had the filter available and would fill the dromedary by filtering while making a crossing. If it was windy and wavy I would some times filter from shore if there were no house boats around. One of the great things about kayaking on the lake is that you do not have to worry about enough drinking water and there is little reason to haul much of it around.