I think this is spot on. But we'll need to wait a bit longer than this December.

In the MtGox chart the pre-bubble pad to the $15 peak is 273 days and then the part that you don't show from the $15 peak to the bubble start is another 137 (to 1.01.2013 - the price then was still about $13.5 and it was about to start bubbling up). The downslide is 133 days - so it is about three times less.

Now the downslide was 413 days (to 14.01.2015 the bitstamp lowest point) or 629 (18.08.2015 - the bitfinex lowest point).

So if we want to have the same proportions the next bubble should start either

about 1200 days after 14.01.2015 - that is May 2018
or
about 1900 days after 18.08.2015 - that is October 2020

Wow that was surprising even for me how far away it gets.