There should be a feature about goal tracking on Duolingo. Let's say I want to go through the whole Spanish program in 6 months: where should I be at week no 4, etc.
I don't think it helps much. I used to learn French here on Duo and on another site. On Duolingo I studied to gain at least 91 points every day to keep my skill coin stacks full, and on the other site there was a goal tracking system. It was less effective. I skipped a lot of days without practising because that had no impact on my statistics. Long-term goals don't work well.
If you keep your coin stacks full every day, you'll finish the course in about 4 months. More than 4 months, if you often redo the lessons or do practice sessions, i.e. gain coins without making progress on the tree.