A helpful member shared that this is a doxycycline antibiotic injection. It's likely you are treating for avian Chlamydia?
I had the unfortunate experience of my whole flock of 7 mixed species parrots coming down with this. While we used oral doxycycline, instead of injection, reading seems both are used. We did the oral for 45 days then I had to repeat , and then do an extended treatment for the budgies. All are fully recovered now. This was a couple of years ago.
They responded differently, but most took a week to really start showing clear signs of improving. I think my vet said it would take about 4 days to start seeing improvement...My very sick( near deaths door ) old lady bird took a month , she was improving but she needed a lot of support, I had to hand feed her 4 times a day for a month then still support feed her I'm thinking at least another month .
Definitely keep communicating with your veterinarian. We had to tweak things when I treated mine .
Also recommend weigh daily , first thing in the morning after first poop. Tracking weight was a great tool in evaluating my birds response. Initially they had all lost weight prior to treatment, stabilized, then slow steady weight gain back . Those thst didn't hit tweaked. Then after the first round , weights started dropping slowly, and budgie hit green stool again and we did the next round .
I hope your baby gets better soon. Support them , log weight and symptoms, keep communicating with your veterinarian, and keep us updated. It's so stressful having a sick bird