I can make some assumptions about my machines, however, that allow me to feel safe doing so. I’m probably signed in to my Gmail accounts for days at a time and across multiple machines here at home. I’m signed in to email - and all my other accounts, for that matter - all day long. If you’re not certain or know that others pose a risk, then absolutely sign out when you walk away. As long as someone else won’t access your machine while you’re not around, or those who can are trustworthy, then staying signed in is generally safe.
