Short answer: around 80%.
As we discuss the numbers that are emerging each day of absentee ballots that have been returned already in various states, I went back to look at the final tallies on this issue from 2016. From the US Election Assistance Commission:
Nationally, by-mail voting constituted 23.7 percent of all votes cast in the 2016 election. Approximately 80.1 percent of absentee ballots that were transmitted to voters were returned and processed, with 1.4 percent of transmitted ballots returned as undeliverable and 2 percent reported as spoiled (e.g., the voter returned the ballot and asked for a replacement).