Saw this list as part of the proceedings of ISC'24 (https://www.isc-hpc.com):
| Year | Computer | Scale | Gap (years) |
| 2022 | Frontier | Exascale | 14 |
| 2008 | Roadrunner | Petascale | 12 |
| 1996 | ASCII Red | Terascale | 11 |
| 1985 | Cray-2 | Gigascale | 19 |
| 1964 | CDC 6600 | Megascale (3 Mflops) |
For the longest time I used to think the gap was around 15 years, but seems to be a tad lower. More importantly, the gap is more variable than I thought. In turn, this makes timing the advent of zettascale somewhat harder to predict.