Gas escapes with heat, so keep the balloons in a cool place
Helium is known to be the lightest gas in the universe. It has a very short life and does not stay trapped for a longer time. You will notice that, when you buy a helium filled balloon, the balloon will start losing air in a couple of hours and deflate it self after one or two days. Sometimes it also depends on what kind of balloons you are using. When using the thick shiny balloons which are available at most supermarkets, they tend to stay for around a week. But these balloons are quite expensive. It is better to use air filled balloons to keep them inflated for a longer time.