How long would it take for atibiotics given intreveniously to work on an elderly patient who was frail and weak?

Can't really say not knowing the severity of the pneumonia, how long the patient has been ill and other health conditions.

Patient was female aged 80, had just recovered week before from Flu, had Flu jab on The Tuesday admitted to Hospital by the Late Saturday night with shallow breathing and temperature.

Again it is hard to say. You see there are many factors that are taken into consideration. Age, previous health conditions, possible allergies to antibiotics...

I am sorry but there really is not a cut and dry answer to your question.

I have a client now who is on dialysis once a week and she has had pneumonia twice in the last six months. She is very well taken care of by us and her family, so there is no telling at all. She seems okay now, but as soon as I hear any hint of a wet cough I will know it is back.