Starting point
Today I'm typing at about 60 WPM using QWERTY, and I can type at 10 WPM using Colemak while having a paper guide taped on my screen. I'm willing to practice 30 minutes per day, while using QWERTY for my job, mainly on KeyBR and Colemak Academy.
My two main keyboards are a Cherry Blue TKL and a 105 with scissor (the Cougar Vantar Black).
Why Colemak, and not Dvorak or any other?
I'm trusting them with the \you can use QWERTY while learning", as it's not as different from the QWERTY keyboard as the Dvorak. Also the switch between mappings are available with:
$ setxkbmap -layout latam -variant colemak $ setxkbmap -layout es
The heatmaps for both layouts are similar, so I don't expect differences overall. I will be happy if Colemak works for me, even if it's only 99% as good as the Dvorak layout.
Progress
Day 1: Qwerty 60 WPM, Colemak 10 WPM.
... [ gaining speed progresively, first muscle memory start to build at day 5]
Day 7: 19 WPM.
Day 9: 22 WPM.
... [ this is one of the first lessons I write full words without thinking ]
Day 10: 24 WPM.
Day 13: 24 WPM.
... [ stuck in 24? ]
Day 14: 26 WPM.
... [ just for the lulz, I did a speed test for the qwerty, and got a 12 WPM with 65% Accuracy. Sad ]
Day 21: 35 WPM.
... [ all keys are "unlocked", which means all of them are above 35 WPM. Now it's just practice to get back to 60 WPM! ]
Day 27: 44 WPM.
Day 28: 42 WPM.
Day 29: 45 WPM.
[...]
It has surprised me that by around day 10-11 I can already start to use the layout. I'm at ~24 WPM with the whole character set, but above 30-35 WPM in the central row. Unlike Qwerty, the Colemak central row is very useful, and you can type a lot with it. Half of the remaining keys are in the Qwerty positions (qwzxcvb and m), specially the bottom row, so despite going slow I can already type without looking at the keys: I have all of them already located, now it's just a matter of building the muscle memory.
While slowly gaining speed while mantaining a ~95% accuracy, above 30 WPM you can't rely on thinking, so the old "qwerty" memory starts to mess with the new memory. My worst swaps are typing "I" for "L", "D" for "G", "G" for "T" and "P" for "R". But it also happens the other way: when typing on a keyboard I don't own, and thus a "qwerty", I type some keys from the Colemak layout.
After 12 hours of practicing, I can already do The Switch. It takes a toll on concentration, as the old casual typing without even thinking about it is not here. You need to focus.
After some hours practicing (a total of 30 hours at https://www.keybr.com, 15 hours at https://monkeytype.com, maybe 5 at https://colemak.academy), I've reached the 60 WPM. I'm not yet as accurate as I was with the Qwerty (99-100%), but almost there (97%). Still I need to focus on typing if I want to type over 50 WPM, but less and less.
Conclusions
Is it worth it? Still not sure, but it feels good. The keys are clearly better positioned and fingers doesn't jump through the keyboard as much as with the Qwerty. The worst keys for me (K, J, Ñ) have low frequency, so they only slow you from time to time.
What would I change. I would first focus on learning the keys at colemak.academy, then go to keybr.com to "unlock" the keys progresively until at least 35 WPM. Finally gain speed at monkeytype.com. The later has a "realistic" mode, where you can backspace and fix errors, mirroring what is like to write a real document. This is a needed practice that the others doesn't allow: they work by blocking-until-correct or deleting-not-possible.
Work to go. I had very quick learned automatisms in the terminal that are gone. E.g. "ls -alrt" or "cat whatever| wc -l", or passwords, that were possible typed at around 100 WPM.
You don't need a Colemak keyboard. Print a small keyboard mapping, stick it to your screen until you learn the keys, and you are good.