it depends on how much work you're prepared to put in.
i research alot and that's why i have a large journal but i also write reflections for most things (ie. class discussions, reflections on major work, new ideas, feedback from teacher/friends/etc, changes in major work....the list goes on).
it's really flexible with eng ext2, so you can go in with as much deep as you want. most of the time my reflections aren't so detailed they're just reminders of what changes i noticed in my major work, reflections on my concept and such.