Omega 3 can get into the blood, and so it is difficult for the alpha linolenic acid, what is the main LCP (long chain polyunsaturated fatty acids) that helps dry eyes, to converted into lipid resolvin e1 to help dry eyes. So you may need 12 to 20 weeks to see the full benefits of omega 3.
And to make sure that you take enough omega 3 in, you should make a highly daily doses of fish oil or flax seed oil or krill oil, among which krill oil is the most effective form of omega 3.
The inner materials in the omega 3 will help moisture your eyes for a while. But it will not last longer. Usually it will be lasted for about several hours, not more than a day. If you have the dry eyes, you'd better eat the food with vitamin C to moisture the eyes.