For the longer-lasting lettuce…
Keep a cool head
Leave lettuce heads intact and unwashed until you're ready to use them and store in your fridge crisper drawer with some paper towels to absorb any excess moisture, so they don't spoil prematurely. Change towels as they become soggy.
Revive lack-lustre lettuce
Restore a wilting lettuce to crispness by soaking it in ice water for a few minutes before you use it. Gently remove, dry, and use.
Lettuce life-expectancy
Generally, lettuce leaves should be used within 7 to 10 days. You probably won't have much luck storing them longer than that. Heads of lettuce have a longer fridge life and can last one to three weeks, depending on the variety and ‘Tight heads’ like iceberg lettuce keep the longest.
Don't toss out a whole head of lettuce when the outer leaves start to look past their prime. Just peel them off, the rest of the head should be fine.
Source: Erin Huffstetler, www.thespruceeats.com