It takes Silver gulls (Larus novaehollandiae) around two years to reach full adult plumage.
Here’s some shots of some of the transitional stages they go through. All photos taken at Caloundra, Queensland.
This bird (above) is transitioning from juvenile to immature. It still has some brown mottled feathers, but not all the way across its back now. The bill and legs are still brown, and the iris is still dark. This bird is probably around one year old
This bird is a sub-adult transitioning from immature to mature. The mottled brown feathers are gone, and the iris is now light, but the legs and bill are still yet to catch up and are a reddy/brown. This one is probably just under two years old
Full adult ready to eat your chips! Note the bill and legs are bright orange and the iris is light and the back is grey