https://www.gamespot.com/articles/you-can-play-nintendo-switch-onlines-new-nes-games/1100-6461834/

Nintendo's online subscription service for Switch has launched, and one of the benefits you get for subscribing is access to a library of NES games. Each of these classic titles supports some form of online play, whether that be competing directly against another player in Balloon Fight or taking turns controlling the action in a traditionally single-player title, but until recently, it had been unclear whether or not the NES games could be played offline. It turns out they can, but there's a slight caveat.

According to the FAQ page on Nintendo's website, the NES games included in the Nintendo Switch Online service can be played offline for up to seven days. That means you'll periodically need to "check in" online every week in order to verify that you still have an active subscription and maintain your access to the games.