What is the etymology of "yonks"?

How did we come to say "yonks" meaning a long period of time?

"I haven't been to the cinema in yonks."

Etymonline has nothing and Oxford dictionaries has:

noun: British informal: a very long time: I haven’t seen him for yonks

1960s: origin unknown; perhaps related to donkey's years (see donkey)

If it has indeed spawned from "donkey's years", when and whereso?

Solution 1:

The OED says the origin of yonks is unknown and has it from 1968 in the Daily Mail:

I rang singer Julie Driscoll... She said: ‘I haven't heard from you for yonks.’

The Shorter Slang Dictionary (Partridge, Beale, Fergusson, 1994) agrees it's from the 1960s and suggests:

Probably from years, perhaps influenced by donkey’s years.

Donkey's years (also donkeys' years) is a play on "donkey's ears" which are long, therefore a long time. The OED has it from 1916 but I found an earlier example in the Australian Trove newspaper archive in "THE WAISTCOAT MAKER" (1905, October 24), West Gippsland Gazette:

'Thank 'heaven fer that,' I says. 'I want to git back to work. It seems donkeys' years to me.'

Solution 2:

The most plausible explanation I have found is here.

It is most likely an abbreviated spoonerism of donkey's (y)ears: yonkeys' dears.

Solution 3:

This article reviews this word, take a look at it. A few quotes from it:

Many people — including Paul Beale and Mr Stuart-Mogg — say they believe it’s a convoluted acronym, formed from “Year, mONth, weeKS”...

...

A few reference books suggest instead that it might be from donkey’s years, also meaning a long time. This sounds quite daft on first hearing, but if you think about it, you can see how the onk of donkey might just have been prefixed by the y of years, perhaps as conscious or unconscious back slang.