Iceberg lettuce is pretty much just water (90%) and cellulose. Almost no nutrients or calories. Low enough so that a quick Google search doesn’t even list them. Loose-leaf lettuce, which is much more nutrious than Iceberg lettuce, has 19 calories in a 52 g (1 cup) serving (http://www.dole5aday.com/FoodService/pdfs/FACTSHEET_Lettuce.pdf). So even for loose-leaf lettuce, you’d have to eat about 100 cups (5 kilograms) in a day, just to maintain a decent caloric intake. I’m guessing that with Iceberg lettuce, you’d actually run a caloric deficit processing it.
So on an Iceberg and lettuce and water diet, you’d last roughly as long as you would on a purely water diet. Maybe a bit less, although the fiber would at least help to diminish the hunger pangs. How long is that? Based on David Blaine’s recent stunt, you’d could last between 3 and 8 weeks under ideal conditions. Here’s a site that actually backs up my WAG with some facts and a list of the damage you’d do to yourself in the process (http://www.radio4all.org/aia/act_hungerstrike.html).