I take a large container with a lid, soak paper towels then lay them in the container, place the lettice mixture then cover that with the paper towels and then place the lid on the container! last two weeks!, NO KIDDING
I think it would be fine for a few hours, if you just made it for your lunch to take to work or school. My kids take salads to school all the time. Just be sure not to take a dairy-based dressing. Use a vinaigrette and keep it in a separate container until you're ready to eat your salad. The greens should be ok until then.
If you are just wanting it to last until lunch it would probably be ok, but not as crisp as when you first took it out of the fridge. I'd keep it in the fridge as long as possible. Over night you are most likely not going to want to use it as it will become very wilted and begin to brown.