Real-world testing by MBL to determine ice holding capability:
Conditions - Cup at room temperature (not pre-chilled), full of ice with the top on.
Indoors (a/c): 9 hours (9pm to 6am). 20% of ice was remaining. Cup and contents cold.
Outdoors on a boat, shaded (83 degrees, sunny): 4 hours (11am to 3pm). 20% of ice was remaining. Cup and contents cold.