Looking at one of mine now .. a yates regular screamer will extend 10", going from 8" to 18". A yates "zipper" has double the number of folds going from 8" to 28".
Not much I know, but last year I saw someone take a lead fall on icy bc, falling on his first screw as he was trying to place his second, the rope caught him about 6" off the deck (he badly torqued an ankle on the way down).
Likely to happen often? No probably not, but I do think about it. Of course we're not supposed to fall on ice anyway right?