The Portugal international broke football's transfer fee record when he joined Madrid in a €94 million deal after six years of service at Manchester United.
He has proved value for money at the Spanish capital club, scoring 60 goals during this season alone, helping Los Blancos win their first La Liga title since 2008.
The 27-year-old says that he is happy with life at the Santiago Bernabeu, but does not rule out a new challenge in the future.
"To be honest, I do not know. In the future, nobody knows," Ronaldo told Sky Sports in an interview which will be aired on Friday.
"I feel very happy here, it is a great club with fantastic fans. It is great, but in the future nobody knows.
"I feel happy here but it one year or two, you never know. A man's head is always complicated, but at the moment I am very happy."
Ronaldo is part of Portugal's Euro 2012 squad where the Iberian nation will meet Germany, Netherlands and Denmark in the group stage of the competition.
Since March, Goal.com has been publishing exclusive extracts from Luca Caioli's new book, Ronaldo: The Obsession for Perfection. This Friday's excerpt looks at when Cristiano won the Ballon d'Or as a Manchester United player in 2008. Don't miss it!