Unarchiving goes from new to old. Keep in mind that the number of posts/topics which are done are done at a rate so that your server does not crash. If you have a lot of topics and a not-so-powerful server, it could indeed take several months to unarchive.
The best way to test if it is unarchiving, is find a topic which is the newest topic which is still archived then run the unarchive task manually, if after a few runs of the tasks, that topic gets unarchived, the task is still functioning. If it is not, please provide us that topic URL and we can look into this further.