The quest pops up at 48. I just made a trial character to test something else and used the hearthstone to skip the tutorial. The quest popped up as soon as ported into Stormwind.
My experience on the beta was that once that quest was in your log, you couldn’t turn on Chromie Time. (Although you weren’t kicked out of it if you were already in it and still below 50.)
So, if you are in that 48-49 range, you could try abandoning “A Chilling Summons” and see if you can access Chromie Time then.
Edit: Tried to abandon the quest on a trial character. It got Chromie’s text to change, but still didn’t let me turn it on. The quest also kept popping back up a few seconds later. It’s possible that Chromie Time doesn’t work with trial characters, and that’s why I got her to give me the timewalking dialog without the option to turn it on, but it’s also possible you just can’t turn it back on once you’ve hit 48. So, I’d call it inconclusive, but likely not good news.
For people who don’t have Shadowlands, this might not be a problem as you don’t get “A Chilling Summons.”