Mike Freer has provided the answer setting out the average time between a claimant receiving their tribunal claim and their first hearing.
In 2008-2018, the waiting times were around 30 weeks, pre-pandemic (2019) it was 38 weeks and as of March 2021, the latest figures, it is now an average of around 49 weeks for the whole process. Although the answer does not expressly say one or the other, I assume ‘first hearing’ includes preliminary hearings, meaning that the average time for reaching a liability hearing in many cases will be considerably longer.
More information can be found here