Few, so 3 years.
But if we wanna do 2 that's still the case.
You don't win a double, by luck and not being one of the best teams in Europe.
You don't follow it up by making the SF again by not being one of the best.
You do quite often.
Last season Madrid were the best team in Spain. That's true. Barca and Atleti were far behind for various reasons.
But they were not even close the top4 in europe, even despite managing to eliminate them in knockout ties.
In 2021 they reached CL semis, yet they were not even close to the top4 that season either.
Cups are unreflective of best teams more times than they are. Abundant historical examples of that.
The best team in Europe every season is not determined by who wins CL, but by a number of factors such as how dominant the teams are in their domestic and european games, what football do they play, etc
Until the real Super League comes (which will be a league, and not a cup), we won't have an indisputable metric which team is the best in Europe every season