How long to recover after chest/respiratory infection?

At the start of feb I gradually went off my food and felt generally unwell. I felt a cold coming on but managed to fend it off before it took hold but it left me with a cough. A week or so later the cold came back but this time it was really bad to the point where I could only breathe through my mouth and still the cough. After getting over this I still felt unwell, no energy, tired, no appetite, but just carried on going to work but around march 10th my appetite completely went to the point where I couldn't even force food down me and a few days later I was really ill. I went to the doctors and he diagnosed a respiratory infection and prescribed me 5 days worth of amoxicillin 500mg 3x daily which didn't seem to really do much, I went back and was prescribed 5 more days of the same and a cough suppressant, he also made me have a blood test which came back ok. I picked up a little but during that time i was unable to eat for 5 days which really scared me. I am now sort of back on my feet but still very little appetite at all and have had to start drinking a nutrient drink, I have no energy which I put down to not eating. How much longer is this likely to last and should I go back to the doctors with regards the appetite?