(I'm assuming you meant 'mark' there......)
But it depends on the exams... There are three groups of standardized tests. First is the state-wide tests. I only have the experience of one state's standardized tests (not explicitly saying, but I've never gone to great lengths to hide my general geographic area...) which I think are the quickest. The teachers grade the exams of the school, so in theory it should be about the same amount of work as if it had been assigned in-class. Not to mention there are deadlines. (Our state English exam took place over two days, and before the exam started on the second day, my teacher told me my grade on the first half, so it shows how fast that was graded
)
The SAT and ACT only have one essay, and it's pretty short; no more than two hand-written pages. It takes 3 weeks for the scores to be posted online. It's only one essay per student, but since it's so many essays--not to mention lots of multiple-choice--it takes a bit longer.
And lastly, the APs, which take the longest... A month and a half. IIRC, they send all the AP exams to the AP Exam HQ (I think maybe in Texas) and grade them there. There can be quite a few tests per student, and more of the test is essay-writing (compared to SAT/ACTs at least... might depend on the AP and state exam). Plus the essays are usually a bit more complex than the other tests, and the rubric is a bit more complex for the APs, all of which contribute to a longer time to grade.
...Phew. I'm not sure there's a more boring topic than the grading technique of standardized tests ^^'