We go for 8 days and purchase on the first day. The balloon lasts all week, and we are able to pass it onto another family upon departure in excellent condition.
Our last few trips, we have driven. That balloon is crazy! We buy on the first day to enjoy through the trip, then it made the drive all the way back to Ohio and then I swear it lasted like another 6 months!
Mickey balloons seem to last forever. If you're flying out of Orlando International, you can take your Mickey balloon to the Disney store in the airport, where they will deflate it for you. You can refill it with helium at any party store, when you get home.
Luckily, we're close enough to drive, so it would just be in the car. If they last that long, then I'll have to consider getting one next time we go. We always start our trips in the Magic Kingdom. Each time we walk in for the first time, I'm very tempted to buy one. But I presumed they deflated in a week or so and I decide I'd rather spend the money on something else the kids would like, such as a dessert or a pin.