I thought a bit and found that the cycles are indeed getting longer.
First cycle duration is 367 days from the halving date to the top. Halving date was 28th of November 2012, date of top was Nov 30th 2013.
The second halving was on July 9th 2016, the top on Dec. 17th 2017. Cycle duration thus 567 days.
So: Every cycle is now either 159 days longer than the previous one, or 1.43x longer, should this lengthening ratio continue.
This would put the top for this cycle to March 27th 2022 (+159 days each cycle) or June 4th (x1.43 each cycle).
I think the top will be at the full 4.236 fibo extension at 300k. I made an overlay of the previous rally from August 2020 to May 2021.
The result fits pretty well with the fibo extension and also with the timing.
Let's see if this is correct.
If yes, most altcoins will see their top earliest in April 2022, or even as late as July 2022.
We will soon find out if this theory is correct :)