How long we should give breast milk to ensure the breast milk given are the best quality? Heard that after 1 year the nutrient & omega are lesser.

No definite answer. Normally, most breast feeding for various reasons is stopped around 4-6 months by most moms. However, it would be best to try to get to 6 months at least because an infant's immune system doesn't fully mature until 6 months of age and breast milk has protective antibodies in the milk to help protect the infant. Nutrients in breast milk are dependent on mom's diet for the most part. Eat well.