All life sentences are indeterminate.
This means that you will stay in prison until you are considered safe to be released. However, there are a small number of lifers who have been told that, because of the particularly dreadful nature of their crimes, they will stay in prison for the whole of their lives. This is the most important factor. Of those released, the average length of time served in prison on a life sentence is between 13 and 14 years. But some people serve less than this and some serve much longer.