Do they expect everyone to finish? No way. The test is designed to weed out those who will not be suitable for their field of choice.
Should you set a goal of finishing all the questions in the time allotted? You had better believe it! Any questions that you don't answer are wrong, and they will hurt your scores. The better your scores, the more competitive your package will be.
Remember, there are hundreds of other people out there that want to do the same thing you do. You have to beat them out, and you have to prove that you have what it takes. The ASTB is only the first step in a long process - don't stumble on it.