Primary Teachers: I need your advice, please!

I’m preparing to help a K-8 school administer and score on-demand narrative writing assessments.  We’ll be using the on-demand writing performance assessment prompt from Writing Pathways.  I noticed (on page 22 and again on page 182) all K-8 students are expected to complete the assessment in 45 minutes.  As someone who only taught upper elementary grades, 45 minutes seems like a long time to expect students in grades K-2 to sit and work on one piece of narrative writing.  I’m sure some kids can use work diligently that entire time, but I’m wondering if the vast majority will…

I was hoping to get some guidance from primary teachers about the time frame you think is appropriate for an on-demand writing assessment, given in September, in grades K-2.  How much time do you think K-2 students should have to complete an on-demand narrative writing assessment?  Please share your thoughts by leaving a comment.  THANK YOU!