How long can you use the results of an Invisalign Itero scan?

I'm 45 years old and my dentist has scanned my teeth 3 months ago with the Iteroscan. She now says that I must NOW (this minute) give her my permission to order the set Invisalign Full aligners because, she says, if we wait any longer my teeth will have shifted position so much that this scan will be useless and we will have to redo it. I'm waiting for the okay of my health insurance. That will take about 1 more month. Do the teeth of an adult shift that much in just 3 months?