NOTHING is "24 Hours" in Buenos Aires... except for some Kioskos.
Grocery Stores: ~8am to 10pm (Sundays & Holidays until 9pm)
Banks: 10am-3pm (Monday through Friday ONLY)
Gyms: Usually close between 10pm & 11pm M-F, Saturday they might close for siesta and then ~7pm or 9pm, most gyms are CLOSED on Sundays