MackMum, I was actually told it by a breast feeding counsellor, not sure where she got the info from, but it really made sense for me as my ds#1 was very big (10.5kg/23lb) at 6mo & I was pumping 240mls at 5.5mo & taking it from a bottle... I did notice that my milk was a lot more creamy after that 6mo point.
I think anything over about 100mls constitutes being hungry... when they gulp it down it tends to be very hungry... a baby that is "crying for a feed" is actually over hungry, the first hunger sign tends to be things like sucking lips & rooting... just like with sleep, crying is the "last sign" if that makes sense.
Spitting up (from my experience with both boys) is often related to growth spurts, something about the changing of supply & for some reason I often found the days I tried to "chokka" my boys I often had worse waking... with ds#2, I was much more laid back & he slept better
It is normal to feel less full towards the end of the day too.
reading jenifer's post & link, I wanted to say 6mo is a huge stage developmentally & so it is hard to work out whether it is a feeding, developmental or other reason... being my nature, if I couldn't settle my boys in 5mins, or they weren't settling, I fed them, if they refused, I could rule that out... the way I worked, others may not agree, but for me I tended to think it was better to feed & all get sleep than be awake trying to work out whether I should or not & at what point was it hunger etc