No more than two years.

http://www.ukba.homeoffice.gov.uk/visas-immigration/settlement/returning-residents/

You may return to the UK as a resident if:

    you were settled in the UK when you last left; and
    you have been away for 2 years or less; and
    you are returning to live   HERE permanently; and
    you were not given public funds to pay the costs of leaving the UK.

If you have been away for more than 2 years, you may still qualify to return to live in the UK if, for example, you have strong family ties here or have lived here most of your life.

If you have been away for more than 2 years, you must apply for a visa or an entry clearance certificate before you can return here. For more information, see the How to apply page.