hi - me again
It really depends what the writing task is. If its a story and the child gets into it quickly i suspect thay could write a page and a half of a4 paper. If its a letter task then it may be shorter.
dd1 who is in year 10 now was the same as your dd, good at comprehension etc but just could not get ideas down for creative writing. She only scored 98 on the writing but much higher on vr so as all the papers are added together the vr pulled her through into a pass. dd2, who took the test this year, is a good creative writer and scored 120 but only 99 on the vr,
so dont worry if she is only an average writer , the maths and vr can get her the pass.
Do you know what writing level your dd is at school. I was told end of year 5 that dd2 got a level 5 in the sats writing task and a teacher assessment of a 4a for writing. dd1 was at writing level 4b.
Hope this helps, fire away with any questions.