I don’t remember exactly, but I want to say it took me a couple weeks of using it for lightweight things that were mostly alphabetical and less frustrating for me to be slow on (e-mails, Duolingo, etc.), using my normal keyboard for most things. That got me more or less up to speed for “normal” typing, but not programming.
Then it was a week or two of using it as my primary keyboard, but switching back to my old one for anything time-sensitive - if I needed to answer a question in Slack or write code quickly, I switched back. This was a bit painful, and punctuation in programming definitely suffered. I also did have to relearn a couple passwords, since those are so strongly muscle-memory for me, but after a month or two I was back to full speed on them.
It was probably another couple months until I felt completely fluent; particularly brackets and braces ("[]{}") took me forever to internalize, and I still slow down a bit when typing them; but overall I feel more comfortable on it than a full-size keyboard.
One factor may be that I previously used a Model 01 in the office; that was a harder adjustment, but I was also switching back to OSX there, so there would have been an adjustment period anyway due to the modifier differences versus Linux. So my hands are somewhat familiar with the ergonomics and general idea of using thumb keys
FYI, my pre-Atreus speed was typically between about 98 and 105 WPM on various online typing tests; I just got 107 (adjusted) on typingtest.com, including a few numbers and punctuation that are a bit slower.
Honestly, my main advice is to just not force it, and accept that it could take a while. Try to use it a little bit each day to adjust, but don’t feel the need to switch to it cold-turkey until you’re feeling pretty comfortable, and don’t be afraid to switch back temporarily as necessary. That said, YMMV; I have no trouble switching back and forth, but I know some people do.
Also, if anything really bugs you, fix it! I don’t like using the outer keys on the bottom row - too far away from both the thumb and pinky for common use. So I moved quote to function-semicolon (right pinky), and did a couple custom chords for escape, tab, and dash; I actually tend to use either the key or chord depending on where my hand is, which is kind of interesting. I also always hit space with my left thumb, so moved space over there, and adjusted the modifiers (ctrl/alt/shift) until they felt good to me.