I've only had Auryn for about 3 months. From my observations with him, he'll be quite content to sit still for awhile--and when he starts zooming around/getting fidgety I know that he's getting uncomfortable and wants to find somewhere safe to hide (He's usually zooming/fidgeting toward anything that looks dark and safe). I don't really ever have him out longer than 20 minutes. Once he starts getting zoomy, I put him back and he zips right into one of his hides

EDIT: I've also noticed a difference between Auryn moving around to explore and Auryn zooming around because he's getting nervous/uncomfortable.