Chavez, a close ally and admirer of Castro, compared the Cuban leader's attempt to recover from an unspecified medical condition to the 1950s, when Castro was a guerrilla in Cuba's eastern mountains fighting the government he would overthrow.
"Fidel is in the Sierra Maestra again, battling for his life," Chavez said after attending a summit of South American leaders in Rio.
Castro, 80, has not been seen in public since shortly before July 31 when he announced he was temporarily stepping aside while he recovered from an operation.
He has provisionally ceded power to his brother Raul, the 75-year-old defense minister.
Castro's medical condition is a state secret, but Cuban authorities deny he suffers from terminal cancer, as U.S. intelligence officials have claimed. Cuban officials have nonetheless stopped insisting Castro will return to power.
In a speech Wednesday night, Chavez called Castro's situation "delicate" but dismissed as speculation recent Spanish press reports portraying Castro as near death after three failed operations and complications from the intestinal infection diverticulitis.
On Friday, Chavez said he could not give more details about Castro's condition "because I'm not the doctor who's caring for Fidel."
He added: "And if I was, I wouldn't anyway, but nevertheless I can tell you: I don't know when Fidel will die, I hope he lives 80 more years, I hope he lives 100 more years."
Chavez is known for making bold statements without elaborating.
Also Friday, he accused his nation's main telecommunications company of spying on him, apparently at the bidding of the United States.
Chavez, addressing 10 South American leaders gathered at a summit of the Mercosur trade bloc, gave no additional details.
The accusation came less than two weeks after Chavez announced he would nationalize the telecommunications company, CA Nacional Telefonos de Venezuela — CANTV.
The company issued a statement late Friday denying the charges. Brian Penn, a spokesman for the U.S. Embassy in the Venezuelan capital of Caracas, declined to comment.