Quote:
"It will take time to recover from any spell in an intensive or critical care unit (ICU), no matter what the illness. Patients are moved to a regular ward before going home.
Dr Alison Pittard, Dean of the Faculty of Intensive Care Medicine, says it can take 12 to 18 months to get back to normal after any spell in critical care"
Source:
https://www.bbc.co.uk/news/health-52301633
While Boris was never on a ventilator (and so probably not requiring quite so much recovery time as some others), his condition was considered serious enough for him to require ICU treatment, so it's not unreasonable to expect his recovery to be a fairly slow process.