The amount needed to satisfy thirst would normally be covered by 15cl; I doubt v much that you need 45cl ( nearly half a litre ) of water to sate a thirst except in rare cases

it is better, imo, to give out 15cl cans than to do what some airlines do and pour from a larger can into one of the small plastic cups

As far as frequency of serving goes, it isn't hard to pop to galley and get another , even back in economy