When you are using Heaven to benchmark your GPU, it does not need to run for a long time. On average, gamers will tend to run Heaven for around 1-2 hours. This will enable them to sufficiently learn how well their GPU copes under stress. There is no real need to run it for any longer than this.
If you are looking to carry out a short test, this will take only around 30 minutes to do. However, it is not always necessary to carry out long tests, and 15 minutes is often enough time for you to gather all the information that you need.