A regular ballon is usually limp in less than 24 hours and useless shortly
after that. The Maylars are good for about a week I think.

I've had a 12" (30cm) Mylar balloon hold its lift for over a month.

I either case, I
wonder how much is escaping through the knot or filler adhesive?

Sometimes a plastic plug is used in He-filled rubber balloons. Might that work
longer?