As so many of my posts, this one will start with a disclaimer.
Disclaimer: I am a firm believer of "the dose makes the poison" and "we all have to die from something". But I'd like the dose to be low enough to not be poison and I also want to be healthy and sexy in the time that comes before dying. I definitely don't think there's a single dietary component that's hurting everybody's health and sexiness as some seem to do. Fat! Saturated fat! Carbs! Fructose! Alcohol! Meat! Gluten! Wheat! Omega-6 fatty acids! (but it is way easier to focus on one thing at a time so that's what I'm doing)
tl;dr You will never become deficient in omega-6. Never.
I dug up a 2010 paper by the American Heart Association (AHA) where they review whether there might be cardiovascular benefits to reap from limiting omega-6 fatty acid intake (spoiler: they find that the experiments reported in the literature are poorly designed but that there appears to be a protective effect from increasing polyunsaturated fat intake but it's quite modest and only when replacing carbohydrates or saturated fat - and I didn't go through the papers they looked at). What was really useful though is the estimates for how much omega-6 humans need. 0.5-2% of the energy intake (okay, based on babies and stuff but still useful). For a 2000 kcal diet that comes out to about 1-4.5 grams. And somehow the AHA got 14.6 grams when they calculated the average daily omega-6 intake for Americans. So that seems like the average American gets about 3-14 times as much as they minimally need (I don't want to be average though). In addition it seems a reasonable assumption that the AHA estimate is low - read on to find out why I think so. I could however not find any reports of omega-6 deficiency in humans. I found a paper on rats where some of the rats were restricted to 0.01% of their energy from omega-6 in the form of linoleic acid but were otherwise provided omega-3s (in the form of alpha-linolenic acid), monounsaturated, and saturated fatty acids - the control group got 2% of their energy from linoleic acid. The restricted rats developed slightly scaly front paws, weighed about 15% less (not necessarily a good thing), and a few of them lost some hair. At the end of the experiment they were fed some omega-6 and they only metabolised 1% whereas the control rats metabolised 34%. So the group with too little omega-6 reduced the amount of omega-6 they used for energy but were otherwise quite alive and doing fairly well.
Back to the real world. Where might one find approximately 15 grams of omega-6? Most easily from junk food.
15.6 grams if you go to McDonalds and get 6 McNuggets, a 22 g / 0.78 oz tiny tub of Creamy Ranch Sauce, and a medium fries.
14.0 grams if you eat half a can of Pringles (but once you pop you can't stop).
19.0 grams if you eat a standard 3 oz bag of microwave popcorn. Though the metric standard bags seem to be 100 g in which case you get 22.4 grams.
12.7 grams if we make sure to get the most important meal of the day and go past Starbucks and get a medium blueberry muffin and a medium, sorry grande, soy latte (this is not breakfast! This is not an improvement on not eating breakfast!).
Or if you are sucking it down straight from the source the daily average intake is the equivalent of approx. 2 tablespoons of soybean or corn oil, or 3 tablespoons of peanut oil, or 4 tablespoons of canola oil, or 1 1/2 tablespoon of grape seed oil, or 11 tablespoons of olive oil. But then you would have to not eat anything else that contained any fat to not ingest more than 15 grams.
These values are pulled from http://nutritiondata.self.com/ and of course I pulled the things with the highest values for shock value (for the soy latte I used a 1 to 7 ratio of omega-3 to -6 to calculate the omega-6 content). But I'm still missing things like margarine, mayo, cookies, crackers and puff pastries (totally ruined your day there, didn't I?). Take-home message is that an average American (sorry again but it's always easiest to find numbers for the Americans) eats at least 3-14 times as much omega-6 as the AHA estimates that they need and eating more than those 15g/day is really really easy.
"What about people who don't eat junk? You paleo people are going to keel over from lack of omega-6, right? Ha!"
Urgh, my own storytelling device (or whatever it's called) is so stupid that I'm giving myself a headache.
Let's go with 4.5 grams of omega-6. It was the higher bound requirement on a diet of only 2000kcal but since even paleo eaters are going to be eating more than 4.5 grams it doesn't really matter. To get ~4.5 grams of omega-6 you would eat:
8 eggs, or 37 almonds, or 2 pork chops (loin), or 2 small avocados (California), or 1 large avocado (Florida), or 3 tablespoons olive oil. Or said in another way, if we skip the almonds we could get 4.5 gram omega-6 covered with 2 eggs, ½ pork chop, ½ small avo, and 2 teaspoons olive oil. Or our weekly (maximum) requirement could be covered by a 250g bag of almonds.
I'm not saying there isn't a place for seed oils in the modern world. I just think that place is in biodiesel and bioplastics.
PS I was going to add a table with omega-6 and omega-3 content of various fats but I just can't be bothered right now. Just know that
seed oils and things made from seed oils: bad
fruit oils, nut oils, and animal fats: fine
Extra reading: http://chriskresser.com/how-much-omega-3-is-enough-that-depends-on-omega-6
EDIT 21.11.2012 Just fell over this paper, in Poultry Science of all places, which has omega-6 requirement at 4.4g for a 2000kcal diet and an upper limit of 6.6g.