Seems to be about once every 10 years. But it might be accelerating due to speed of technical innovation.
The most recent one has obviously been crypto. Whatever will be next is probably bubbling somewhere on a forum or subreddit with a few people who really believe in it, but most not really taking it seriously. The Paul Graham 'looks like a toy' [1] heuristic is a good one.
The chief requirement seems to be a small, organic but very intense group of people, with a long term view, who are unflinchingly sure that it - whatever it is - will work, eventually.
This seems to be as, if not more important than what the thing actually is.