I first introduced a bottle of FF at about midnight when Harry was about 3 weeks old. It wasn't without guilt and worry though, but came after a good few nights of having so little sleep that I was exhausted. Hubby would give the bottle of FF so that I could get a bit of sleep. I had tried to express, but at 3 weeks could barely get 1oz out after sitting pumping for a LONG time.
We're currently trying to wean Harry off boob during the day and on to FF for when he starts at nursery.