The answer to this kind of question always seems to be - it depends. Well anyway, it depends:
A condition is normally attached stating how long the planning permission is valid for.
Normally domestic planning permission is valid for 3 years.
Outline Applications are valid for 3 years.
New Housing and Commercial development are usually valid for a maximum of 5 years.
Submission of the reserved matters must be submitted within 2 years of the date of the approval of the outline.