Switch Online NES games will only function for 'up to seven days' without an internet connection

If you plan on taking advantage of the many NES games coming to the Switch Online service, be prepared to keep an internet connection going on your Switch. While you can get away with not being connected online for up to 7 days, it seems you'll want to make sure you connect at some point before that week ends, as the NES games may cease to function.

Nintendo Entertainment System – Nintendo Switch Online can by played for up to seven days without an internet connection. If you are unable to launch Nintendo Entertainment System – Nintendo Switch Online, then please try connecting to the internet again.