For iceberg lettuce it forms a head like cabbage would, i believe you harvest it once it forms a head. It isnt like leaf lettuce where it will keep growing new leaves once you harvest the iceberg that its for the harvest but you might have some sideshoots to grow new leaves.
I dont have any lettuce pics right now but I find it easy and fun to grow lettuce!
I use the method of plant heavy then just thin at baby green stage..then ad the mid stage then leave the nicely spaced ones to form mature heads ..then you have lettuce all the time !
lettuce has been reall easy for me. I sow thick and to the cut and come again method. Make sure to stagger your planting so that while one is regrowing the other is ready. I don't do head type lettuce because it would take more care in sowing. Where as leaf lettuce I just sprinkle and water.
Lettuce in general is extremely easy (to me, it is one of the Big Three things that it is stupid *not* to grow b/c they are so easy and so much better and more convenient than storeboughten, the other two being 'maters and beans)
Iceberg (and head lettuces in general) are more challenging than most lettuces, however -- they require extra good growing conditions, in terms of very ample and constant water, and you as a previous poster points out, you cannot do daily partial harvests, you have to take the whole head at once. So it is awful easy to end up with, like, a dozen heads that need to be used in the next week *STAGGER* your planting!
Leaf lettuce is easier, it is more lackadaisical about conditions although you still shouldn't give it a drought, and you can pick a couple of the outer leaves every few days (whatever the plant's growth can bear) to keep harvesting over a looong period of time, ended only by it bolting.