Some people might prefer to hike Mount Batur as solo travel, because you can arrange your own pace and plan during the tour. There will be no time limit s everything will be depend on your own preference. However, traveling to Mount Batur alone is challenging, not only because the track is quite difficult for beginner but also because you are not familiar with the surrounding.
Hiking Mount Batur will require knowledge about the weather, the route, the shortcuts, the location for the photo, the nearby village not to mention an understanding about the local language that make you able to communicate with local people. unless you are confidence that you can manage these things, traveling solo is not recommended; as so many foreign people are unable to find the right track during the trekking session.
Basically, purchasing the tour package on local tourism company is recommended for those who want to hike Mount Batur as single traveler or in group, because they provide you with facilities that you need during the hike.
The guide who accompany you are mostly fluent is several languages, so you can ask question and explanation when it is needed. They are also familiar with local attractions and recommendations that will help you to decide where to eat, to take photo, and to participate in some local event. Instead of hiking the mountain by yourself, having companion who share the same interest will be more satisfying.