Ok. I've asked this question a long time ago when they were babies and the answer I got was, it's preference.
But... I filled a 1.5li bottle 2 days ago and within 6 hrs, he drank 1/3 of the water and now there's about 50ml left.
Now they're 2yo. Sugar takes a 7 - 10 days to finish a 1.5li...