30 cm. latex balloons generally last between 12‐20 hours when filled with helium, and 3-30 days when treated with Hi‐Float depending on various environmental condition (It's a tough world out there, if you're a balloon: sun, wind, rain, high altitudes, and grit are all formidable threats to a balloon's welfare).
Larger latex balloons will fly for longer, up to 1 month.
Foil balloons last up to 4 weeks, and you can add air using a straw in order to maintain the shape of the balloon.