An 11 inch/28cm helium filled latex balloon will last approx 12-16 hours depending on conditions. Hi-Float can be added which will extend the floating time.
Larger latex balloons (16 inch/40cm & 3 foot/90cm) can last for days, again depending on conditions. Helium filled Foil balloons will last for many days depending on conditions. Helium filled Bubble balloons are the longest lasting balloons as they are made from plastic, not latex. In the right conditions, they can last for weeks.