Depends where you buy it from. A lettuce bought from my local chain supermarket will be lucky to reach the end of the week in edible condition, a lettuce bought from my local farmers' market will still be mostly-ok (just take off the outer leaves) by the end of the month
It was refrigerated, and I believe still had the plastic wrap around it, which protected against oxidisation somewhat.
Fun fact: Liz Truss would've lasted an even shorter amount of time as PM had the queen not died on her watch. That shut down parliament for a while. In normal times she wouldn't have lasted more than 3-4 weeks I'd imagine.