On his twitter account, a fan asked @KurtBusiek about George Pérez exit on the Avengers (vol. 3, 1998) series. Busiek replies:
"It was a lot of work, and he didn’t want to keep going like that, on a monthly schedule."
"The in-office bet was that George would be the first of the Heroes Return artists to need a fill-in. He was the last."
"There were four issues he didn’t draw in that run, but he drew four extra-length issues, too. It was a tough schedule."
"He drew most of an issue in the hospital."
"Tom (Brevoort, Avengers' editor) would absolutely have brought in a guest artist to help out, but George said no, he’d do it."
"If I remember correctly, he didn't tell Marvel he was in the hospital until he’d drawn a batch of pages while in a hospital bed."
Pérez last issue was #34 (2000). A big battle against Count Nefaria with the Thunderbolts and Madame Mask involved.
Link: Twitter