For beverages or for drinks?
For soft beverages at a fountain dispenser, I put in about 1/3 to 1/2 ice. When I am drinking soda at home, I do not add ice as I have generally chilled the beverage in the fridge prior to pouring.
Adding ice at a soda fountain is good because I am often taking the cup away with me and I want to keep my beverage cool while I am in the car. This is unnecessary at home as I can pour a half glass that will stay cool while I consume it and then pour another half glass later and both will be cold. Also, a glass that I drink from at home is generally a better insulator than a thin plastic cup I get from a soda fountain. So my soda stays colder, longer w/o ice at home than from a fountain.
For drinks, it obviously depends on the drink, but I like half ice in a Old Fashioned glass w/ my G&Ts.