My first needed more than milk from about 4 1/2 months. I didn't realise, nor did the HV. Tried hard to keep going to 6 months with just breastmilk. He would drink a lot, settle, then fuss after 10 - 15 mins. Stopped gaining weight. Started waking a lot in the night wanting milk.
In desperation I gave him some baby porridge on day (feeling guilty!) and he settled immediately. Happier, slept better at nights, started gaining weight. His poops got a lot nastier though!
Lovely HV actually apologised for not picking up he was one of the uncommon babies who needed more, though I don't think she needed to - I wanted quite badly to keep him on milk.
Second baby did it by the book, he was fine until 6m when we started weaning.
So 6 months -should- be okay, but it's worth keeping in mind that the occasional baby does seem ready for more, earlier. I hadn't heard of Hungry Baby Milk then, or that would have been the next step.
The older has always been tiny though, very low percentile, and the lack of weight gain was a real worry.