Although there is no statutory time limit on detention under Immigration Act powers, the law does not allow indefinite detention. For detention to be lawful there must be a realistic prospect of removal in a reasonable timescale. Individuals are detained for no longer than is necessary. Detention in all cases is kept under regular review to ensure that it is both necessary and appropriate.
The number of people held in detention as a result of the revocation or dispute of their refugee status is not held in a format which is reportable. However, the number of people who have been detained and have raised an asylum claim can be found on table dt_05 of Immigration Statistics; https://www.gov.uk/government/publications/immigration-statistics-year-ending-march-2018/how-many-people-are-detained-or-returned#immigration-detention