If you’re the main breadwinner, you will want to keep your family in something like the style to which they have become accustomed. If you’re a carer, then you want to provide cash for professionals to take over because you’re not around.

Until your dependents are financially able to look after themselves, 20 years is about right for most people or until dependents complete full time education.
For most people as they get older liabilities diminish and savings accumulate, consequently one takes the place of the other and therefore reducing the necessity for life insurance. One could also argue that this in its self creates another problem, one of Inheritance Tax, and here life insurance cover, written in trust can be particularly useful.