Most botanical gardens have a dozen or so, but they only bring out one that’s blooming when they think it’s a good year to fundraise. While it may be true that any given one only blooms every 20 years or whatever, in practice they’ll bring one out when the stock market is at its peak, or at a big anniversary year or whatever.
It's ~7 years give or take a few. I used to go see blooms at Foster botanical garden in Honolulu whenever they had one which was about annually, never experienced an extra fundraising drive. I don't really get why you'd choose to spitball on the topic.