There are technically Life Imprisonment sentences which are to last for the entirety of a persons life.
However, there is often (as in like 99%) a parole set out by a minimum term (which is set out when sentenced).
On average Parole comes after 15 years. Sometimes it can be longer.
It is possible to be released on Parole from a life sentence and then commit a crime and gain an additional life sentence and be released on Parole later.
Crimes like Murder now have a manditory Life Sentence.
It is possible to be given a Whole-Life tarrif/order, in which case you recieve no chance for parole during your sentence.
It's common for people to be released on parole, or earlier for "Good Behaviour".
Personally, I find the entire thing disgusting that a person can be given a life-sentence and be released in just over a decade (or less). If you've been given a life sentence then it should be for life.
If you commit a murder at the age of 20 you can be released at the age of 35. You're likely not even half-way through your life-span.
It's a common belief that Life-Sentence means 25 years in England and Wales. I don't know where this statistic comes from, despite being wide-spread, but it's close to the average of 15 years. (According to
Wikipedia)