At a guess, I would say we will see 27" iMacs with ARM in about a year. The iMac Pro will take somewhat longer.
From the Apple press release:
"Apple plans to ship the first Mac with Apple silicon by the end of the year and complete the transition in about two years."
If they literally mean one Mac product going AS (Apple Silicon) by the end of 2020, the easy route would be the Mac mini, but the Apple fashionista route would be a laptop.
Apple also mentioned there are a few more Intel products in the pipeline, I would bet iMacs & iMac Pros! These are the onlyy current products that are somewhat languishing on Apples website. Tech specs does not even state which generation of Intel CPUs are being used (because they are OLD), when Apple has no issue stating which gen is on the most recently updated ("NEW") products. ac mini will not get a new Intel CPU, it was recently spec-bumped & will next get an AS APU. Same for 'low-end' laptops, straight to an AS APU. All MacBook Pro / Mac Pro products will be on the back half of the transition; MacBook Pros, then a new Cube, then the iMac Pro (which is just the Cube specs), and finally the Big Chungus Mac Pro chassis will go AS, but not actually until sometime in 2023 for shipping product (probably announced / paper launched in Dec of 2022).
So iMacs & iMac Pros will get some sort of Intel spec-bump update, because someone is always buying an iMac & the iMac Pro will be an interim sale for someone who is balking at dropping serious cash for a current Mac Pro but needs something before the transition is complete.
New / Spec-bump Intel iMacs & iMac Pros - before end of 2020
AS-based MacBook "I'm living on it..." - Q4 2020
AS-based Mac mini - Q2 2021
AS-based iMac - Q4 2021
AS-based MacBook Pro - Q2 2022
AS-based Cube & iMac Pro (same baseline specs) - paper launch WWDC 2022 (two year transition) / pre-orders Q3 / shipping Q4 2022
My thoughts towards future Apple Silicon intended for a Mac Pro:
32 P cores / 16 E cores / 64 CPU cores / 32GB HBM2e UMA / 500W APUzilla