Actually, while I hate to out and out contradict people...Most babies were born at home until the turn of the century (1900's). Even then, it wasn't until the 1930's that doctors began antiseptic practices (like the simple washing of hands....Doc's LITERALLY used to come upstairs from the morgue without washing their hands. Not all, my own grandfather was a huge proponent of what is today called Standard Precautions). But (I even looked up the numbers at the CDC, so I wouldn't just be remembering http://www.cdc.gov/mmwr/preview/mmwrhtml/mm4838a2.htm) it wasn't until the 1940's that the majority of babies were born in hospitals. Prior to the late forties, the GREATEST number of babies born in hospital was 55%....but far more commonly fewer than 30%. And the vast majority of infant deaths within 3 days of birth, were those that were born in hospitals. Until the past few decades it was actually waaaay safer to have a baby at home.
The idea of keeping babies in wards (which has now been discarded for MANY reasons) was developed at approximately that time, and the idea of feeding babies on a schedule was actually a combo of what NURSES were capable of preforming in a ward with dozens of infants, and the fact once they left the hospital...that with a war on, we had Rosy the Riveter Mum's who could only nurse on their cigarette breaks...and then *poof* comes the godsend formula...which meant a) they didn't have to leave their shift, and b) that you had to be reeeeeaally careful about not feeding babies too much or too often because it would make them sick. Sometimes lethally so.
So, really, the whole scheduled feeding thing...around for about 70 years....not hundreds...much less the 60,000 years or so of human development. For the past 60,000 years...we have tons of archeological & historical evidence, in addition to the infant care practices that we can observe in many modern day hunter/gather tribes, as well as in many other more agricultural societies. Mostly it involved having your infant strapped to you...so that you would be able to continue you everyday activities.
Back to your question though:
Normal for a six month old is anywhere from every half hour to five hours. Guaranteed that ALL babies eat more and more frequently just prior to a growth spurt, or after a lot of activity.
Think about it...are you hungrier if you've been doing a lot of exercise all week...or if you've been sitting around? When you were growing a baby were you hungrier that when you weren't in a "growth" spurt. And WE were only growing 7-10 POUNDS. They're doubling their weight every couple of months. Can you imagine doubling your weight in 3 months? Being 10-12 feet tall by next year???? Yowza. Puts things in perspective a bit.
As you may have guessed...I'm a BIG proponent of feeding on demand...not because of history, but because it's what makes SENSE. Eat when you're hungry. Sleep when you're tired. If I can see that a child is hungry, or tired, I see it as my responsibility to feed them, or get them to sleep. Not as their responsibility to NOT be hungry because it isn't "time" yet. They can learn self control a discipline in a few more years (heck, they'll be learning it for the rest of their lives). Teaching them that there is a scarcity of resources (that they can never eat when they want to...or get love & comfort when they need it...just doesn't seem to make any kind of sense.
Babies know what they NEED
Children know what they WANT
Adults have AGENDAS.