There's no standard. Different companies produce different products with different levels of purity and different amounts of omega-3 fatty acids. They also all seem to recommend different daily dosages. Before you buy anything, be sure to read the label. Look at the amount of fatty acids in it. Look at the amount of EPA and DHA, the fatty acids associated with health benefits, listed (they may not actually be listed, like with the stupid Costco brand ones I got

). The amount of EPA and DHA are what's really important, not the amount of fish oil or omega-3's. I would suggest that you look up the studies on whatever benefits you want to get from fish oil. The amount you should take will depend on the amount that the studies show to be effective for that particular benefit. You will most likely be taking more than the recommended dose on the bottle, given that those recommendations aren't really based on anything, and multiple fish oil "pills". Take them evenly throughout the day so that you don't have too much in your system at once.
Be careful with fish oil though. Like anything, it can be harmful in too high a dosage and can cause intestinal bleeding. It's best to consult a doctor before you really try to work with the rather high dosage of omega-3 fatty acids that are correlated with decreased depression and aggression.