"When the time comes, I'll move on. It won't be a difficult decision to make," the 39-year-old said ahead of Thursday's Nations League clash with Croatia in Lisbon.
"If I feel like I'm no longer contributing anything, I'll be the first to leave," he added, citing the example of his former team-mate Pepe, "who left through the front door" after announcing his retirement from the game last August at the age of 41.
The five-time Ballon d'Or winner, who has been playing for Al-Nassr in Saudi Arabia for almost two years now, failed to score as he appeared at a record sixth European Championship this year, with Portugal exiting in the quarter-finals.
In spite of the criticism he received in the press Ronaldo said he "never considered leaving the national team" and he has retained the backing of coach Roberto Martinez.
"People's expectations of the national team were too high," said Ronaldo, adding that the bad times in a footballer's life "allow you to evolve".
After Thursday's meeting with Croatia, Portugal remain in Lisbon to face Scotland on Sunday.
