But that's my point - it's not linear. It could speed up, or stop, or anywhere in between. If your PD is 3 years ahead of the current VB, you can't assume it will be another 3 years until it's current.
The only target is the number of visas issued each year, but there's no target for the Visa Bulletin dates, as it will depend on how many people are ahead of you in the line.