So I went and looked up Question Time online - and the mighty Wikipedia came up. As all the rest of the first page of searches were to articles complaining about him getting so much airtime, I took it.
According to their list of episodes Farage has the 11th most appearances on the show (34). Which he's managed over 19 years (from first appearance in 2000) at an average of 1.7 appearances a year.
Which is not unreasonable for the leader of 2 parties that have consistently been able to poll between 5-15% for the last 10 years - and done considerably better in European elections.
Although that 1.7 a year is higher than anyone else the top 10 - with the top 3 all managing 1.6 a year.
Above him on that list are 2 Tories (Ken Clark and Michael Heseltine), 2 Labour (Roy Hattersley and Harriet Harman), 4 Lib Dems and Shirley Williams (who appeared as Labour a couple of times then became SDP then Lib Dem).
The preponderence of Lib Dems is a similar reason to Farage being on a lot, there are fewer of them, though they've been invited to most episodes since the 80s. Hence all the senior ones with long careers being at the top. UKIP had few other senior people, so we got lots of Farage. The Brexit Party is even more of a one-man band.
Then the 4 Labour and Conservatives were all senior ministers / shadow ministers with long careers - the two Labour ones having been deputy leaders of the party.
So Farage's appearance rate really isn't unusual. He became a European MP in 99 and has been on QT from 2000-2019 less than twice a year since - while he's been UKIP leader since 2009 - and is now Brexit Party leader.