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]