Back in the breast pump days, on through the formula days, and now into the milk milk era, we've stuck pretty close to a 45 minute/1 hour time limit for the kid's milk. After a bottle or a cup had been sitting for that long, we'd throw it out, or if the kid wanted to [finally] finish it, we'd usually replace it.
[Actually, with formula, if she'd started and then stopped on a bottle, we'd usually just make a new one rather than put a "contaminated" dose back in the fridge.]
But when grandma saw me pouring out half a sippy cup of milk and refilling it over Thanksgiving, I got to hear about how they grew up drinking milk that was kept on the porch for days, in buckets, even. Room temperature was not the enemy of the people that it has become, apparently.
Now I wonder if I did the right thing throwing out the cup of milk [solids] I inadvertently left in the diaper bag over the weekend. Maybe I should've just given the kid a spoon.
"breast milk can be stored at room temperature for up to 10 hours." [breastfeeding.com]
But no more than four hours is better. [ivillage]
I stick with the four hour rule myself, and I've even gone to putting leftovers back in the fridge to use later, if it was only out for a short time. I haven't noticed any reactions from the little one, so I suspect 4+ hours out is totally fine.
Back in grandma's day the milk that they were getting was probably unpasteurized. This meant that it was not protected against the nasty bugs that could (infrequently) be harbored in milk. This is a larger potential problem in our age of modern factory farming, which is why it is damn near illegal for you to get your hands on unpasteurized milk unless you actually own a cow. In addition to killing off nasty bugs, pasteurization does a bang-up job at killing all of the non-toxic bacteria in milk as well (e.g. lactobacilli and other bits that would make her milk eventually turn into yogurt, etc.)
With the non-toxic bacteria killed off the milk is now an open growth medium for whatever happens to land on it. These new bacteria do not face any competition from exiting bacteria in the milk, which previously would do things like change the ph in the milk over time ("souring" it) and otherwise try to protect their turf against these new invaders.
You are better off dumping the milk, but are probably not taking a huge risk by not doing so.
Um, how old is Grandma? Pasteurization was created by Louis Pasteur in 1865, now granted it wasn't widely used until 1900. But even still, that would make disapproving Grandma at the kitchen sink 105yrs old? I think Grandma's attitude probably comes more from a depression era mentality where things just weren't considered as disposable as they are now. But back to the milk, I agree with Matt, 4+ is fine, I've given it to my kid with no adverse reactions.
[Grandma just asked me for your address; I suggest you lay low for a while. heh. She just turned 60 and grew up on a farm, so when I hear milk came in a bucket, I assume it was coming straight from the cow. As to that depression era thing, though, I grew up brainwashed into thinking that every square of toilet paper over three that I used was an unforgivable waste of resources. And long after free weekends and unlimited minutes, great grandma still hustles off the long distance calls as quickly as possible. -ed.]
I go with the four hour rule myself for most things, although with milk, formula and breast milk, I limited it to 2 hours at most.
Usually, the milk will go back into the fridge for one later use. If she still doesn't want it or doesn't finish the second time, then it goes.