I would focus on one skill at a time and not really put a limit on the time. You may have half of your students ready to move onto a new skill and the rest not there yet. I would continue to introduce new skills and then the students may need individualized instruction. I find that writing is the easiest time to individualize instruction, because everyone is busy writing, and you can easily work one on one.
Do you have to have a writing grade? We don't grade for writing, mainly just prepare for state tests. I only grade grammar and conventions.
Hope this was helpful
