The 2020-21 numbers in this CNN chart are as of Monday, December 28th. I excluded additional races from 2008 and 2018 as they added little information, having been run at the same time as the races with more voters and being of less note.
CYCLE OFFICE RUNOFF TURNOUT SHARE OF NOV. TURNOUT SHARE OF ELIGIBLE VOTERS
2020 U.S. Senate* 2,337,477 47% 32%
2008 U.S. Senate 2,137,956 57 34
2018 Secretary of State 1,473,904 38 20
1992 U.S. Senate 1,253,991 56 26
Obviously, if the vote stopped there, while the number of voters would be impressive, as a percentage of the turnout and of eligible voters would not be astonishing --> but it didn't stop there, of course.
As of this morning, 3,001,017 have voted, according to Georgia Votes. That's 18% behind the November election through the equivalent date. If so, it's already over 60% of the November turnout, setting a new record. It is also over 41% of the total eligible voters, blowing away the record from 2008's runoff.
That's without election day turnout, which while smaller was not inconsequential in November.