Since I don't know your specific product, I would say try to get up to at least 1500mg of the omega-3's a day to see a difference in joint comfort. This may anywhere from 1-4 teaspoons of your cod liver oil. It should state how much of the omega-3's are contained in each serving on the bottle.
Make sure however that you take your cod liver oil with food, especially a fat containing meal because cod liver oil often has vitamin A and D as an extra, and since these two vitamins are fat soluable, taking them without a meal will be harmful in the long run.