How long do you need for a party balloon to stay filled? A normal party balloon will hold hydrogen just fine if the relevant timeframe is on the order of a few days. How do I know? I run an organic chemistry laboratory. My students use normal party balloons (much cheaper than balloons sold by lab supply companies, but equally effective) to set up reactions in a hydrogen atmosphere. Place the reaction flask under vacuum. Backfill the flask by connecting a balloon that was filled from a hydrogen tank. Voila... a reaction under an atmosphere of hydrogen. If everything is well sealed, then the party balloon will not leak an appreciable amount of hydrogen in a day. Mind you, these are party balloons being exposed to harsh organic solvents. In a typical home environment, they should hold up even better.