The statutory determination period for validated planning applications, which local planning authorities should not exceed, is 8 weeks for straight-forward planning applications, 13 weeks for unusually large or complex applications, and 16 weeks if the application is subject to an Environmental Impact Assessment (EIA).
Failure to determine the application within these deadlines means that the applicant can choose to appeal to the Planning Inspectorate on the grounds of non-determination. Local planning authorities have to inform applicants of these rights.
See “Determining a planning application” on the government website which sets out the process and expectations on planning performance and decision making.