When is nec usually diagnosed, should I be worried if my baby is constantly fussy and has blood in his stools?

Early. Nec is mostly common in premature infants who are hospitalized in a intensive care nursery. Infants rarely get nec at home. The most common cause for bloody stools in a child at home would be a milk protein allergy and may respond to a change in formula. Always consult with your pediatrician first.
First few weeks. If your baby has bloody stools, you should take him to see his doctor. This is not normal.