The question we hear most at the counter is how long a balloon will stay up. The honest answer depends on the balloon, and on the room.
An 11 inch latex balloon filled with helium floats for about half a day indoors. Treated inside with a gel, the same balloon lasts about three days. Foil balloons hold helium for a week or more, and the big foil numbers often last two weeks (sample).
Heat makes balloons pop and cold makes them sink, so keep them out of a hot car and away from radiators. For a party on Saturday afternoon, collect latex on Saturday morning; foil can come home on Thursday.