Robert A. Caro knows that his fans are anxiously awaiting the fifth and final biography in his celebrated series The Years of Lyndon Johnson. But for now, he’s not saying how long they’re going to have to wait.

“It doesn’t matter how long a book takes, what matters is how long a book lasts,” Caro told the Associated Press in an interview after being asked when the follow-up to The Passage of Power might be finished. He said it’s the standard answer he gives to all curious readers.

Caro’s series of lengthy LBJ biographies have been some of the most acclaimed in recent American history. He published the first book in the series, The Path to Power, in 1982; The Passage of Power came out 30 years later. The other titles are Means of Ascent (1990) and Master of the Senate (2002).

Between them, the books have won two National Book Critics Circle Awards, two Los Angeles Times Book Prizes, a Pulitzer Prize, and a National Book Award.

Caro said that the COVID-19 pandemic has complicated his plans for the fifth book; he had to postpone trips to Vietnam and Austin, Texas, where he needs to do further research for the biography.

He said he’s up to 1967 in the fifth book, which will cover Johnson’s presidency and his final years. The former president died in 1973.

Fans of Caro’s book series will almost certainly have to wait a long time to read the fifth LBJ book. In a 2018 interview with the AP, he said he was still “several years” away from finishing the biography.

Michael Schaub is a Texas-based journalist and regular contributor to NPR.