The colour of history. Apple's history.
The original transition took 7 months. I think Cook will have it as a badge of honour that this transition will take no longer. (It hasn't been explictly said.) But I'll say, 'No longer than a year from the 1st consumer Mac ARMs.'
eg.
'New' (ofc) Macbook ARM. (Covering the Air, Macbook 12 and Macbook 13. Expect consolidation.)
'New' (ofc) iMac 24 ARM.
'New' (...) Mac Mini ARM.
All from Dec' to June at the latest. In that order. So, in 7 months. The 'consumer' transition is over.
I'd expect the AS15 chip (after the A15 goes into the iPhone next September...) to...
'New' (...) iMac 32 ARM.
'New' (...) Mac Pro ARM.
Fall of 2021 (early 2022 at the latest...) to complete the 'Pro' side of Mac ARM.
(Macbook ARM 1st...this late fall so...) Specificially iMac 24? January 2021 at the latest. The consumer Mac ARMs all perform similarly on Intel. So a single AS 14 clocked according to laptop to iMac form factor...will be pretty much the same 8 core plus 4 low power core chip with Apple GPU. Tuned according to form factor. There's no reason, given that, the iMac 24, Mac Mini and Macbook ARMs couldn't all be released this fall. But they'll probably stagger it to early 2021.
The 27 inch iMac (32 inch?) will be probably (like the Mac Pro?) dependent on the AS15. So fall next year. A 'year' from September/fall of the pending Intel iMac. (Which we're still waiting on.)
Azrael.
[automerge]1594317305[/automerge]
Anthony13, I think...
That's just about it.

We can't rule out Apple debut of the 24 and 27 ARM and Intel together. But I get the feeling there will be a distance of 3 months between them. With Apple taking the full time to the turn of the year to get the iMac 24 ARM launch (and Big Sur software...) right. ie to make it 'sing.'
In the meantime. We'll get the supply constrained/component dependent launch of the Intel 27 iMac in September.
Apple using up the remaining months (productively, Big Sur is yet to release...) of the year to slip in the iMac 24 ARM in December. It could be the Macbook ARM that is released this fall with the iMac 24 given the squeeze until early 2021.
Azrael.