We use no specific writing curriculum in 7th grade. Essays associated with daily Reading/History assignments are the most frequent writing assignment. You should also have your student write about any special events attended or unique occurences that effect the family. Examples are trips to the opera, classical music concerts, or museum. Attendance at debate/speech competitions, science fairs, or homeschool days at the state capitol all provide great topics for essays, poems, or narratives. If your family watched a political debate or thought provoking movie together, have your student reflect on these and write their thoughts and questions.
You may assign a daily writing exercise, or something that takes more preparation and would require several days to complete. We typically assigned a writing task that could be completed in a single day (about an hour total). The assignment is checked the following morning. Grammar or spelling mistakes must be corrected. Style is also discussed with the student.
Looking back, I believe we should have done more weekly assignments and worked through the process of brainstorming, outlining, drafting, and final write more thoroughly. A multi-week final history project was required which taught many of these ideas, but ongoing work on larger weekly tasks would have been beneficial.