In June 2016 approximately two per cent of roads operated by Transport for London (TfL) had a 20mph speed limit, which equates to 18km of carriageway*.
Currently, as of March 2020, approximately seven per cent of TfL’s roads operate with a 20mph speed limit, which equates to 62km of carriageway*. This number includes early delivery of my Vision Zero commitment to introduce 20mph on all TfL roads in central London, by May 2020 (http://content.tfl.gov.uk/vision-zero-action-plan.pdf).
I do not expect this to change before May 2020.
*the lengths quoted refer to the carriageway length in each direction