1/16th*
Basically lost in the first knockout round every single time, think Lyon eliminated them thrice including that first season in 09/10.
Actually, it was on 1/8th. We met in the middle.
Yep, they lost that year he went there. Since then, I believe they won it 4 times, and reached semis in every other season.
Since Madrid sacked De Bosque (symbolic end of the Galactico 1 era) their UCL results were:
2003-2004: eliminated from Monaco in away goals (quarter finals)
2004-2005: eliminated from Juventus 1-2 (1/8th of finals)
2005-2006: eliminated from Arsenal 1-0 (1/8th of finals)
2006-2007: eliminated from Bayern in away goals (1/8th of finals)
2007-2008: eliminated from Roma 4-2 (1/8th of finals)
2008-2009: eliminated from Liverpool 5-0 (1/8th of finals) -> probably their lowest point in many decades
Ronaldo signs
2009-2010: eliminated from Lyon 2-1 (1/8th of finals)
2010-2011: eliminated from Barcelona 3-1 (semi-final) yipes, they finally won a KO tie
2011-2012: eliminated from Bayern in penalties (semi-final)
2012-2013: eliminated from Dortmund 4-3 (semi-final, Ronaldo is top scorer in the tournament)
2013-2014: WON, Ronaldo is top scorer
2014-2015: Eliminated from Juventus 3-2 (semi final, Ronaldo is joined top scorer)
2015-2016: WON, Ronaldo is top scorer with all-time record
2016-2017: WON, Ronaldo is top scorer
2017-2018: WON, Ronaldo is top scorer
Ronaldo leaves
2018-2019: Eliminated from Ajax 5-3 (1/8th of final)
2019-2020: Lost the first tie at home 1-2 from Man City (1/8th of finals)
Obviously, Ronaldo was not the only factor there, after all it is a home game, but the record with Ronaldo and without Ronaldo (both before and after) is arguably the most different it has ever been in the history of a club/player. It is absurd how good and consistent they were when he was there, how bad they were before, and how bad they became after.
Saying that him putting Madrid to the top was not a challenge cause Madrid were always great, is plain lie. It isn't even not telling the entire truth, it is just lying for the sake of propagating whatever agenda is there.