Hi Susan:
Your personal reflections are invaluable, because they are yours, and that makes them them unique. They provide both a record and a method of expressing your thoughts. Journalizing does take time. But it doesn't have to take a lot of time. I've heard "5-minutes immediately after class" more times than any other. But some will spend 20-30 minutes a day. I think a good way to guage how much time to spend is how much one refers back to their journal. If you find that there's more internalized reflection than the concrete "this worked and this didn't", kind of comments, you might be spending too much time - you can use a diary for the internalized reflection thoughts.
I think good journals are those that capture ideas, comments, improvements needed, impressions - all for reflection for use in future classes. As you say, it pays off positively over time.
Regards, Barry