Reading time: 5 segments. 

The reader of the scroll is protected from all forms of acid, up to a damage limit of 20 hit dice or a time limit of 9-12 turns (d4 + 8), whichever comes first. 

Thus, the scroll would provide safety from three separate breath-weapon attacks by a black dragon of smallest size (normally 6 HD of damage per attack), with a small amount of protection yet unused — assuming that the attacks all take place before the time limit expires.