I have a 3L hydration pack and when I did the trek in July I had about .5L left when I got to trail camp on the way up. The next day when we left trail camp I filled up and I actually ran out of water halfway down the switchbacks. It would have been nice to have an extra liter.
I drink a lot of water when I hike but the best advice I have read was to take your hydration pack and use it to fill a water bottle with a known volume (nalgene bottle) and that way you know exactly how much water you are drinking and it wont be a surprise when you go to drink and its gone.
But like most have said, water is not a problem on this trail until after the switchbacks.
