I think it's somewhere between day 6 and day 12.
I had covid in March last year. I started symptoms the same day as Boris Johnson.
The weird thing with covid is that it starts mildly and gets worse for the best part of a week, before it starts to get better. That's kind of unusual because usually viruses start bad and get gradually better.
What I found was that I was just super tired for a few days, then got too weak to sit up, and then couldn't eat or drink. By about day 7 I was really struggling to breathe. I think it was about day 8 that I started to feel a little tiny bit better and then it was maybe about day 10 that Boris Johnson went into hospital (or thereabouts)
Anyway, as I understand it, it's around days 7, 8, 9, 10 that things are really tricky, if it's going to go that way. You want to start to see things picking up by about days 8, 9, 10 really to feel as though things are going the right way Good luck.