The folks at Duolingo have published an article (and matching blog post) about how much time test takers should be given to complete writing tasks. Their research suggests that shorter tasks are just as useful as longer tasks in terms of reliability and validity.
This is a controversial topic among people who take the time to look at the sorts of tasks that are included on tests of English proficiency. It has generated some discussion.
Nowadays, both test makers and test takers seem to favor test forms that are shorter (in duration) than those used in the past. Since long essay tasks require a significant amount of time to complete, they are less popular than they used to be. Recall that last year the 300(ish) word “Independent Essay” task was dropped from the TOEFL, in favor of a 100(ish) word “Academic Discussion” response, meant to simulate a message board interaction. Research provided by ETS indicates that the shorter task is just as useful as the longer one it replaced.
A separate (but related) controversy relates to how closely test items should resemble real-world tasks carried out by students in the course of their future studies. The move to shorter writing tasks means that newer tests often include items that do not simulate real academic work. Some people find this problematic. Some do not. Yet others argue that the tasks on more traditional tests never actually simulated this sort of thing in the first place.