Large Mylar balloons last for about three to four days before beginning to deflate. They last another two weeks in a slightly deflated condition. The balloons typically float for about four weeks. Small Mylar balloons deflate faster and will float for about two weeks.
Mylar is a brand name for a polyester film used for its reflectivity, stability and strength. It was developed in the 1950s. In addition to balloons, Mylar is used in food packaging, telescope filters, engineering drawings, compact disc packaging, as insulating material and in solar and marine applications. Other items made from Mylar include kites, confetti, measuring tape and adhesive strips on teabags.