How much to do?

How much can they comfortably do in a reasonable time limit?  I’d set a time that seems in accordance with their capabilities and current level of attention, and then choose passages that can be done by the student in that time.  I don’t know, of course, how attentive your students are.  I like copywork to take about ten minutes.  Dictation, I sometimes allow a little longer as the students are generally a little older.  I rarely start dictation at age 8.  Dictation work should include the passage preparation and the actual writing of the passage; my older students can take 20 minutes or a bit longer but I start much shorter for beginning dictation students.

Passage preparation should include going over the passage carefully, spending time on any word the child may not be able to spell until they can spell it BEFORE the dictation begins; I also teach any useful punctuation or other conventions necessary for understanding how to punctuate the passage, and we look up any words we don’t know.  I also point out style issues as the students get older.  We do this the day previous to the dictation and then review anything especially the spelling of trouble words, the next day before beginning dictating.

I do dictation once a week at the beginning, then move up to twice a week where we leave it until junior high-ish age when we need less assigned copywork.  I do copywork every day until  handwriting is well settled and legible.  Yes, learning cursive counts as copywork.  Do letters and words until the child has enough variety to do passages, then begin with short passages and work up as proficiency and writing speed improve.