I assume if stored properly impressions should be good for some time. I’ve heard that weight gain/loss can change ear size/shape. As for me, I’m as fit as I was in college (I wish). On a related note, I make my own impressions as al the audiologists I’ve contacted in NYC have quite $150+ for rear molds/impressions. I don’t recommend making your own impressions though unless you know what you’re doing.
In NYC check Dominic Servedio - got my impressions form my Phantoms from him for abt $60, perfect fit first time, and he works with UE and 64 Audio as well I believe. Think he launched a separate website purely for IEMs but can't remember it.
https://advancedaudiology.net/