Helium balloons

[empressali]

I didn't really know which section to put this in, but hopefully someone can help.
How long do helium party balloons stay inflated? Not the foil ones, the normal latex ones. We have a 50th birthday party for someone at work next week with not much time to do the room decorating. I wanted to do the ballon decorations in advance.
Any advice would be much appreciated.