One of the factors that determines the time to finish the race is the course itself. A hilly course is more challenging than a flat course.
Running Base and Training
Another factor that determines the time is the amount of training and effort put in before starting the race. If you plan to run a half marathon with very little training, the impact of the distance on the body is going to be huge. You will feel tired and weary towards the end. You may have to slow down considerably towards the end of the race to finish it if your training is insufficient.
Proper training ensures that a good running base is developed before the race. If the long runs are done correctly for training, the distance for the half marathon seems a lot easier.
Another factor that determines the time is the pace of the person. Some people have better pace than others. For a beginner, any pace between 10-14 minutes/mile is good. This leads to a finish time of 2:20 to 3 hours.
The first and foremost consideration for any runner for a first half marathon should however be to cross the finish line and not worry about the time. The finish time can be improved once the first half marathon finish line has been crossed.