Question to the Home Office:

To ask the Secretary of State for the Home Department, what estimate he has made of the average time taken by his Department to make a decision on an asylum application in each of the last ten years.

The Home Office does not publish data on the average time taken to make a decision on an asylum application in each of the last ten years, nor is that information available in a retrievable format.
However, the Home Office do publish data on the number of pending applications that have been awaiting an initial decision for more or less than 6 months. This data can be found at Volume 1, as_01 of the Immigration Statistics December 2018:

https://www.gov.uk/government/publications/immigration-statistics-year-ending-december-2018/list-of-tables#asylum