Mylar ballons can hold the helium for about a week before deflating.
latex balloons can loose their helium in about 12hrs or so, and be hanging by the ground in 24hrs.
Now since Disney puts a clear mylar over the latex, it holds it up for about a week as well.
to deflate the mickey/minnie mylar, carefully untie or cut the string. Then place a straw through the hole and slowly deflate it. If you do it this way, you'll be able to reinflate it whenever you'd like. You can even take it to a Party City and they'll do it for like 50cents.
the mickey head balloons, there's nothing really you can do besides put a hole in the bottom and let it slowly leak out. Even if you let it deflate on its own, I don't think it's possible to reinflate it.