E-ink only actively uses power while it's changing the content on the screen. Assuming you want a static image, e-ink can outlast that easily while using virtually no power. Mine lasts for five years now even though it's "on" the entire time (because even when it's locked, it displays a wallpaper).
Power isn’t the main issue I’d be concerned about re: “lifespan” for signage. I wonder more about how it handles long-term UV exposure, day-night temperature cycles, changing humidity, etc.