We had this car seat for our DS as we got the whole travel system. He's now nearly 14 months and we've recently moved onto a Joie 360 spin one.
If I remember correctly, there were inserts for what I think would be a very small / prem baby (our DS wouldn't have fit in it at all from newborn - he was 8lbs).
Then with the normal headrest and inserts we kept until he was about 5 months but it was a squeeze towards the end, to the point that he hated the car seat because of this and would have a total meltdown wherever we were going / just put him in a bad mood.
At the time I don't think I could find any guidance as to when they could be removed. Only that they should have the headrest until they can support their own head or something like that (dont quote me on that I just remember thinking it was really vague and unhelpful!).
Removing the body insert was fine as he filled the car seat no problem, but I always felt like his head didn't have much protection if we were in an accident, but it is extremely snug with it in.
We switched to a Joie Juva baby car seat for a bit as we found the headset situation better and the 360 spin we have now is fantastic tbh. It goes from newborn and I kinda wish we'd just got that from the get go for him!
Hope that helps a little bit!