Also, how does flax seed oil compare directly against fish oil? For instance, does it take twice as much to equal the same benefits of fish oil, or can you not get some of the things from flax seed oil that you can get from fish oil?
I'll try to look up references on this, though I'm sure you could search for it on Google and get decent results.
Basically, the omega-3 oil in flaxseed oil is Alpha Linoleic Acid, abbreviated ALA. The omega-3's in fish oil are Eicosa-Pentanoic Acid (EPA) and Docosa-Hexanoic Acid (DHA), which are longer chained fatty acids.
The body has enzymes to desaturate and elongate ALA into DHA and EPA. It's fairly complicated, because these same enzymes also elongate the omega-6 fatty acid called Linoleic Acid, abbreviated LA or sometimes LNA, into one or more long-chain omega-6 fatty acids. The only one I know off the top of my head is Arachadonic Acid, ARA, which is an essential fatty acid for baby's to grow and develop properly. As far as I know, ARA does more harm than good in adults, so keeping ARA levels low is a good thing. And keeping DHA and EPA levels at least at some minimum level is a good thing.
So what to make of all this? Well, first of all, there are two things to consider. Since the ALA and the LA have to compete for the same enzymes to get turned into the useful longer-chained fatty acids, it's important to ingest a certain ratio of these two fats. The ratio doesn't have to be exact, but recommendations I've seen range from 1:1 to 4:1, LA:ALA. In other words, if you eat 20 grams of Linoleic acid (omega-6 from plant sources), then you should eat at least 5 and not more than 20 grams of Alpha Linoleic Acid (omega-3 from plant sources). The typical American gets an omega-6 to omega-3 ratio of about 20:1, which means that hardly any of the ALA they eat will get turned into DHA or EPA. And if they aren't eating fish or organic graze-fed beef, etc., or taking fish oil or krill oil supplements, etc., then they aren't getting much DHA or EPA either.
Now, for the more important part. How much ALA should you take every day? Well, as I said, the ALA has to be converted to EPA and DHA. The conversion process is slow, so it may take as much as 3-30 grams of ALA to get just 300mg of DHA/EPA.
That's a very wide range, but it's because you have to take into account how much omega-6 (specifically, linoleic acid) you eat as well. The more LA you eat, the more ALA you'll need to eat as well. For example, foods are typically fried in vegetable oils high in LA, but almost devoid of ALA. Taking flaxseed oil rebalances the ratio.