It depends on so many factors:
age of the bun
if the infection is active, if so how active
if the rabbit is underweight
if there is any scarring on the lungs or congestion in the chest
how damaged the immune system is
if the rabbit is on antibiotics
the length of time the rabbit needs to be under
the surgery being performed
Theres a whole lot more.
The only person that can really tell you is the vet who has been treating the rabbit to date.