I'm not sure if this is actually the case. There have only been two bubbles resulting from halving events. That's not enough information to extrapolate. And if it was enough information the market would price that information in early like it always does.

I'm seeing 4 year cycles and I'm seeing 18 months between bull markets.
I don't see any kind of lengthening on any kind of measurable scale.
It only takes one short cycle to bust up that entire theory.

All this being said: a lengthening cycle with diminishing returns is the best we could possibly hope for. That would be a legendary environment for alts to thrive. A stable Bitcoin at a high price combined with more time is exactly what this market needs.